Best ask other mobo makers directly whether legacy booting/VBIOS is supported with Intel's graphics on 500-series (or 400-series) combined with 10th-gen CPUs. If you get more info, it'd be interesting to hear.
I'm not entirely sure, but it appears this may be Intel's fault rather than Asus's. The Asus FAQ is misworded a bit. I think "UEFI" shouldn't be in that sentence, and what they're saying is that the 500 series doesn't have a VBIOS. And with no V(ideo)BIOS, there's no legacy booting.
Some old and new info mentions Intel getting rid of legacy BIOS in 2020:https://www.intel.com/content/dam/suppo ... tforms.pdfhttps://www.anandtech.com/show/12068/in ... fi-by-2020
Someone with an i9-11900 (typo in first post?) on an Asrock H470-chipset system was asking about legacy boot support:https://community.intel.com/t5/Processo ... -p/1317255
Then quoted Asrock's reply:
The integrated graphics of 11th Gen CPUs does not support legacy mode
What's puzzling is that he says Pentium G6400, which is a 10th-gen CPU, can
boot in legacy mode.
A theory: maybe because the graphics there is UHD 610, and not UHD 630 like the higher-up 10th-gen Cores?
On your TenForums thread
someone said he successfully booted Tails in UEFI mode. I think trying to figure out how to get Tails to UEFI-boot is the better path to follow.
Another option: Despite your initial plan, do get a standalone graphics card (if you can find reasonable ones, nowadays). But make sure it comes with a VBIOS.
And hope there would be no more legacy-boot hindrances or incompatibilities from the mobo UEFI/BIOS side.