What desktop computer would you recommend for maximum freedom?

The biggest thing you will run into is the graphics chip and wireless card. You can use this page to find hardware compatible with Linux libre: https://h-node.org/hardware/catalogue/en

The tradeoff is just that much of this is older hardware.

Would one not be safe if using any GPU using the free drivers that come with something like arch?

I'm not sure of the criterion and ive never tried to go to this level of freedom so I'm not claiming anyone to be wrong, but hardware would only do what the driver is capable of right?

How much less freedom would one have running Arch with an AMD RX 7900-XT using the free drivers compared to Parabola with any AMD card listed on https://h-node.org/hardware/catalogue/en ?
- so AMD is likely going to be one of your best bet because they open source their drivers. However YMMV because I don't see that specific card listed, that doesn't mean it won't work but you might have to do some sleuthing to find out for sure. You can always install third party drivers as a compromise until you replace the hardware down the line. I know that's not something anyone wants to do but sometimes that is the only thing you can do.
  
  How much less freedom would one have running Arch with an AMD RX 7900-XT using the free drivers compared to Parabola with any AMD card listed on https://h-node.org/hardware/catalogue/en ?
  
  So that is sort of a philosophical question. I don't think it could be actually quantified with numbers. If you run Arch that will be the standard kernel which has a number of binary blobs by default that are non-free. It also has a repository list that contains non-free packages. These are all things that some advocates frown upon. However if you don't use any of those non-free hardware the binary blobs just sit there and do nothing. If you don't install non-free programs you don't have non-free programs installed. There may be some non-free things installed anyway that your could route out and remove. This is the benefit of a free OS like parabola someone has done all that work for you and they have a linux libre kernel that removes the binary blobs.
  
  There are some people that this just does not work for because of performance, battery life, unsupported hardware etc... Is it better to use a mostly free OS than a proprietary one? Absolutely! is using a completely free OS better than using something like Arch? Likely but you are starting to split hairs at some point. That is just my opinion. Some people are definitely of the attitude that it's free software or nothing which is a completely fine stance to take but it might not work for everyone. More freedom is always better so getting the best system possible with the most freedom you can is still better than using all proprietary.

As far as I'm aware, you can't buy a prebuilt computer that ticks all of those boxes. System76 has Coreboot-supported laptops, but they don't offer it with their desktops because it's nearly impossible. And you can just forget about Libreboot, since it barely runs on anything. Neither is exactly ideal for modern systems, though...

The best you can do is build your own, but even then, it still requires the Intel FSP. Installing your own distro would be as simple as plugging in a flash drive with the installer when you're done.

I found out that Thinkpenguin offers to disable Intel ME for you if you pay $30. I asked another European vendor if they could do that for me if I pay more, because it seems difficult to do that on my own.
- Ever thought about checking Purism:
  https://en.wikipedia.org/wiki/Purism_(company)?wprov=sfla1
- I don't see any mention of Coreboot or Libreboot, though.

It's sort of a shame to think that it's so difficult to have truly free hardware. But at the very least there are many projects working on making it as free as possible. If older hardware is an option, you can get pretty close. ThinkPads with Coreboot come to mind.

I don't know but maybe check out pine ( https://www.pine64.org/ )

I have an ARM Linux laptop from them and there are no intel or m$ (even possible?). It's a cheap computer though, not for gaming for example.

The one major gripe some Free Software advocates will have is with the firmware. However there is a question of diminishing returns with some of this stuff. On some level going ultra staunch is more of a philosophical choice than a technical one.
Neither Coreboot nor Libreboot support the ARM CPUs in the Pinebooks.