As per the title really. I’d be looking to pick one up second hand and use it on EndeavourOS.
Is there a really worthwhile boost in performance moving to the 6700XT, or should I wait a bit longer to get something else higher end? I will not consider Nvidia as a Linux user.
That is for you to decide.
The way you decide is to go to a reputable review website like https://www.techpowerup.com/review/?category=Graphics+Cards&manufacturer=&pp=25&order=date and select one of the reviews of a relatively recent, unrelated card, for example the XFX Radeon RX 7900 XTX.
For this specific site, you then go to the “Average FPS” (or “relative performance”) page and look for your models you actually want to compare to in the list. Since you picked a relatively recent review, you can also see newer cards, so you can also see how much of an upgrade you’d get with newer cards you maybe haven’t considered yet.
In this case, the average fps of the 5700XT is 49.2, while the 6700XT gets 60, an improvement of ~22%. Now, you either know what 49 and 60 fps look like, or you go into one of your older games and use an fps limiter to limit to those fps values and play a bit and compare.
In any case, the final decision is up to you and no one can really tell you what you think your money is worth.
Edit: of course, this assumes your CPU is not a bottleneck. These tests always go with the most powerful CPU. You haven’t said anything about yours, so I assumed it wouldn’t be a bottleneck.