Made me look at used XT3 and XT4 pricing, and, that, unfortunately, is still pretty high. The release of the XT5 has not brought that down.
I sold an XT-3 with 16.5K shutter actuations that was in "excellent" mechanical and cosmetic condition to one of the usual retail suspects for US$570. They will probably resell it for a little over US$900. (I purchased it new for US$1499 back in Nov., 2018.)
There is a list of Fuji lenses that will give benefit on the X-T5. Google will show it. When I spoke to Fuji UK they said all their lenses will work but those on the list will work best without saying what they mean.
I don't see a huge difference in intrinsic image quality between the X-T4 and the X-T5 with any lens. Obviously, a lens that resolves exceptionally well can eke out more detail than one with more modest resolution, but based on my early experience with the X-T5 that's just as true at 26 Mpx as at 40 Mpx. The reason I traded up from an X-T3 is that I wanted a second body that had sensor-based stabilization since the Fuji primes lack lens-based stabilization.
I haven't had an opportunity to compare the autofocus
performance between the X-T4 and the X-T5, but focus
accuracy when using the machine-learning features (e.g., face and eye detection) is noticeably better with the latter, probably because the X-T5 processor has more compute cycles.
I'm sure the extra resolution will come in handy on occasions when I want to crop an image aggressively, but the difference between 26 and 40 Mpx strikes me as incremental. It's important to remember that there can be quite a bit of variation between samples of the same lens model, so generic comparisons of which lenses work better with the higher-resolution body are only generalizations.