Gateway isn't necessary, as we have the capability to determine zero-g effects on the human body on the ISS

It's not the zero-g effects, it's the full dose of galactic cosmic rays and testing "storm shelter" shielding for (potential) solar proton events.

Realistically, SLS/Orion will be ready to fly a lunar mission before SX Starship is wrung out and ready to fly a crew in cislunar space, Starship will be harder and take longer than you think.

"No numbers for the real designed-in deep space radiation protection..."

Crew Dragon Capsule dry mass: 9,525 kg
Crew Dragon Capsule habitable volume: ‎9.3 m³
Orion CM dry mass: 9,300 kg
Orion habitable volume: 9.0 m³

"and propellant mass..."
Propellent mass is essentially irrelevant.
Orion's SM fuel is separated from the CM by the Crew Module Adapter, and since (at the scale of the s/c) solar protons are isotropic, Orion's CM doesn't get any significant radiation attenuation from the SM fuel.

Both s/c have some fuel tanks surrounding their habitable volumes. However, the Crew Dragon's much larger propellant tanks for the SuperDraco Escape system actually surround the "well" in the base of the cabin where the crew would huddle during a CME event. Advantage: Dragon.

Again -the improvised radiation shelters in both Orion and Dragon surround the crew on five sides with boxes of supplies, with the sixth side protected by the s/c heat shield. Since the SpaceX Dragon retains its Martian return velocity capability, the Dragon heat shield is a much thicker (better) shield than the Orion's shield. Again: advantage Dragon.

This series of comments is ostensibly about Zurbin's "Artemis 8” Lunar mission using Dragon vs. waiting for(ever) for the EM-2 (aka Artemis II) SLS/Orion mission.
In reality, the radiation protection offered by these two s/c is very similar, with a slight advantage offered by Dragon's structure.
The BIG difference in crew radiation exposure is due to the mission models. The mission model for EM-2 is the comparatively "high risk Russian Roulette Lunar mission"
Artemis 8 uses earth orbit rendezvous to assemble the elements, then performs a direct TLI and gets "the hell out of Dodge" rapidly.
In comparison, EM-2 (Artemis II) using the sad SLS interim cryogenic propulsion stage will do a double orbit pass through the lower Van Allen radiation belts at 35,000 kilometers. I'm sure the crew will be fine, I'd go without hesitation.

James, you don't know what you're talking about -as usual. Draft horses -flogging? What?
What are you even doing commenting on The Space Review?

"Your hand waving away a LEO mission design Dragon's spacecraft's inherent radiation shielding didn't offer any direct radiation comparison with the deep space design and inherent radiation shielding of the Orion spacecraft"

There's no "hand waving", just the simple facts that anyone can see. The document you linked to, 2016WRMISS-RGaza.pptx (WRMISS 2016) is clear as a bell... to everyone but you.
The chart at the top of page 3, SPE Response Scenario, of WRMISS 2016: 2006:
-Safe haven partially in the bays
-Cabin reconfigured to optimize shielding
-216 kg of dedicated radiation shield

2016:
-Safe haven completely in the bays
-Cabin reconfigured to optimize shielding
-0 kg of dedicated radiation shielding (emphasis added)
0 kg. Zero. Zip. Nada. What part of ZERO don't you understand? Really James, what part?

As the WRMISS 2016 makes crystal clear in both text and graphics, Orion's "Designed for Exo-LEO, Current baseline improved the crew protection" is the result of added radiation sensors (and their software) plus extra bungee cords. That is it Moon Miner, sensors and extra bungee cords.

"LEO spacecraft are not necessarily designed to be beyond LEO spacecraft because to do so would add unwanted mass that is not needed or desirable for LEO missions". As usual, there's unintentional irony in your comments. The original 2006 Orion designed for Project Constellation, was destined for LEO use as crew transport to ISS AND beyond LEO flights to the Moon. But in order to shave weight for the underperforming Ares I re-design, NASA jettisoned that 216 kg of dedicated radiation shielding in the original Orion airframe. In short, the 2006 Orion actually had"...unwanted mass that is not needed or desirable for LEO missions". You are such a rocket scientist James.

Bottom line: the not necessarily designed to be a beyond LEO spacecraft SpaceX Dragon, only requires Orion's radiation sensor suite and extra bungee cords to be functionally equivalent to Orion in the crew radiation protection department.

“…Dragon that was designed for short duration missions in the lower LEO radiation environment have the same heavy radiation protection for astronauts as the Orion”

Whatever gave you the idea that Orion is designed with some sort of “heavy radiation protection”?
In the event that Orion’s crew is caught in a solar proton event (SPE), the plan is to gather up boxes of the ship’s provisions (food, water, clothing, poop, urine etc.) and form a “storm shelter” where the crew can huddle with the boxes on five sides with the s/c heat shield and service module facing the sixth side.

Here's a short YouTube clip in which two chipper NASA gals show Orion's rad shielding: https://youtu.be/70GrihLXmSs

A lunar mission Dragon crew that’s caught in a SPE, can do exactly the same improvised storm shelter procedure as an Orion crew.

Starship could explode:

1) depressurization of one of the main tanks
2) failure of the LCH4 header tank
3) through one tank and through the bulkhead
4) mix in the enclosed tail-to-tail
5) good old fashioned ramming

So what.
These are not hypergolic propellants, you need an ignition source for a deflagration (not explosion) to occur.
So on top of your fantastically unlikely impact event, you have to somehow have an even more unlikely ignition source.

This is just more James Moon Loon FUD.
Three of Four times a year a Progress s/c with over 3000 lbs of actual hypergolic propellent docks with the enormous ISS in LEO, but I don't see James spouting FUD over the possibility of an accident with that.

"Currently, Blue Origin's upcoming New Glenn has a clean and green hydrolox upper stage. Maybe the New Glenn will eventually have a hydrolox first stage, too. Blue Origin's Blue Moon Lunar Lander is also powered by a propellant efficient hydrolox rocket engine…"

“clean and green hydrolox upper stage…Blue Moon Lunar Lander ” You bring up hydrolox upper stages a lot James, soooo you’re worried about CO2 polluting outer space and the moon?? Really??

“New Glenn will eventually have a hydrolox first stage, too”

NO James, it will NOT eventually have a hydrolox first stage. New Glen, like Vulcan and Star Ship, are clean sheet, modern designs using the ideal fuel for affordable, reusable launch vehicles -methane.

"If you knew half as much as you pretend to know, you would know that Japan's upcoming Lunar mission supporting H3 Heavy is a clean and green all hydrolox launcher."

The H3 Heavy is fantasy, there is essentially zero chance the Japanese government will fund this extremely expensive development effort for a vehicle with no commercial uses.

Liquid hydrogen for Japan’s JAXA launchers is made by the Iwatani Corporation in Sakai. The hydrogen is made from natural gas feedstock using the high-temperature (~2000F) steam-methane reforming process. The Liquid hydrogen made in Sakai, is transported 960 miles to the Tanegashima Space Center by convoys of insulated diesel trucks.

The making of hydrogen in the reformers, cooling it to cryogenic temperatures to liquefy it, then hauling to the Space Center is extremely energy and CO2 intensive. A quick back-of-the-envelope calculation shows that from 5 to 7 times more CO2 is put into the atmosphere by your so-called “clean and green hydrolox” launchers than by the new state-of-the-art methane fueled launchers.

The 5 to 7 times more CO2 from the Hydrogen fuel does not count the filthy Solid Rocket Boosters that the parallel burn hydrogen launchers need to get off the ground or the huge amount of energy and materials used to manufacture these throw-away rockets.

And NO James, steam reforming natural gas is the only source for industrial-scale hydrogen production. Electrolyzing water takes fantastic amounts of electricity and there are NO plans by JAXA, NASA or ESA to invest $billions in electrolysis systems for a few, antiquated, government-subsidized launch vehicles.

Welp, I'm envisioning the (most likely) sort of failures similar to what happened in 2018 where there were some ≈4" rips and tears with less than taut cables. You, on the other hand, are promulgating a 100%, total failure scenario in order to prove your point. So it goes.

"...attention to tiny numbers of protesters,

Well said.

Sorry, for those in Rio Linda: the propellant for the reaction control thrusters is used to desaturate the momentum wheels. Even if everything goes perfectly with the s/c, the supply of hydrazine is a life-limiting consumable.

I would be so painful to see such an expensive, productive "discovery machine", simply run out of gas. This, of course, opens the door to a future servicing mission of sorts. As JWST runs low on propellent, a modified Mission Extension Vehicle (MEV) could dock to the s/c bus and extend its life by providing reaction control.