Buildapc

I think you're right on not being able to find a dual slot 7800XT, the original OEM versions (which tend to be the cheapest but also the least flashy/most compact bar specific SFF variants) are long sold out now. It can certainly be a struggle to find smaller GPUs these days, with many third party beefed up cooling solutions requiring at least 3 slots. This is where the CPU cooler is often the point of compromise, but it's not impossible to find more narrow GPUs.

Another option could be the 9070 / 9070XT which is a step up from the 7800XT, I chose the Powercolor Reaper model myself due to only being 2 slots wide so I would have room to play with in my T1 case. I believe the GPU market is still pretty dire assuming you're in the US, so it could be quite the task to even hunt one down for a reasonable price/close to MSRP unfortunately. There are plenty of 9800X3D/Terra builds on the SFF sub, so it might be worth reading through a few of the threads to get an idea of build considerations, temps, etc from those users, here's an example: https://www.reddit.com/r/sffpc/comments/1j68leo/yet_another_fractal_terra_9800x3d_9070_xt/