@colourofsound
Given that I use a Leica Q3 (which replaced my X100F, and ok is a bit of a luxury item, but its broadly equivalent), I also shoot motorsport occassionally. For the later I went through lots of options in my head/research these included
Sony A9 (original, preloved) + GM100-400 (expensive glass) but good FF AF performance
XH2S + XF100-400 - Fast AF and more reach as the sensor is APSC
But I settled on a pre-loved (ex-
@Jelster) Gripped X-T3 and a preloved XF100-400 (total cost less than a pre-loved X-H2S) - I felt that this gave me the best bang for my £££ (given that it was occassional use and not my main camera) - I have since looked swapping the X-T3 out for an X-H2S for faster AF, but most of 'missed' shots are down to my capabilities not that of the X-T3. I've been put off by the X-H2S in that I would need to buy new exepensive cards (and card reader), its a totally different battery system to the X-T3 (I have 6 X-T3 batteries and I can get through 4+ in a days shooting motorsport, 5000+ images), and that it might be hard to sell on if I didn't like it (its a bit niche even for Fuji)
I am happy with 26Mp sensor using the 100-400, and not felt the need for more resolution giving me cropping options for more reach (I could always add the 1.4TC), but if you were going for a lens with less reach maybe cropping and hence higher resolution becomes important. Although the tracking options and AF speed on the X-T3 isn't as fast as the current cameras, its a bit of a sleeper with high burst frame rates. I need to improve my technique(s) to make the most of the AF before worrying about better AF. The X-T3 got a significant AF firmware upgrade during its lifetime giving it similar to X-T4 performace.
Despite bouts of GAS I can't see myself changing out the X-T3 (or buying any other lenses), and if I did I would probably go FF
In your shoe's I would consider the usage, if its going to be a main camera spend the £££, if its occassional use, look at alternative more cost effective options.