I bought a Fuji X-T30 back in 2019 as an "upgrade" from the Canon 60D because I wanted something smaller that I could shoot my kids sports with (they were much slower at the time.) At the time the mirrorless landscape was much different and the Fuji seemed like a good option. In retrospect, for my use cases I probably should have gone with Sony at the time (Canon and NIkon mirrorless option were too new back then.)
Nowadays I'm still shooting my kids, but now they are older and faster and I'm finding the Fuji 55-250mm isn't cutting it when they are in covered/indoor arenas when I'm usually maxed out at f/4.8 . I also like to shoot aircraft as I'm in the flight path of Moffett air base and there is always interesting aircraft i can catch on approach.
I'm disapointed there are still not many great telephoto options for the x-mount. However, I do like the controls and don't really want to learn a new system at this point. So I'm looking at the 100-400mm and the 50-140mm lenses and wondering if I could get by with just the 50-140mm plus the 1.4x and/or 2.0x teleconverters. Should I even entertain the 100-400mm? Alternatively, I still have my Canon 60D and could go for something like the 70-200mm f/2.8, but that setup is quite large/heavy. I was also never happy with the autofocus on moving subjects with that body.
What's wrong with your 55-250? It gives you a 375mm FF equivalent reach, which you can double by cropping in post to 750mm. With today's NR in Lightroom and other editing software, high ISO is not an issue.
O have the 90mm f/2 to supplement the 55-200 when lighting low. The 90 issosharp you can easily crop down to the FoV of the 140 end of the 50-140, and even beyond and you have an extractor over the zoom as well as better sharpness.
