Up until recently I have run two systems side by side, Canon full frame and Olympus m4/3. The weather here in the UK can be pretty dull at least half the year and I found that full frame for wildlife was more usable than APSC sensor bodies The FF Canon body I was using for the past 4 years was a 1DX , the lens being Sigma 150-600C. An excellent combination but very heavy to carry around all day. The body delivered lovely images but had two major pitfalls for me, size/weight and noisy shutter.
To that end I have sold/part exchanged the Canon body and Sigma lens and bought a Sony A7Riii and the 100-400 GM with the 1.4x teleconverter. As all bird photographers say "you can never get close enough" but the 560mm reach coupled with the 42mpx sensor has satisfied me and enabled me to crop more than I felt able to with the 1DX sensor. The AF is excellent even if it's not up to the standard of the A9, I've had no issues with it.
I've also bought an 85mm lens for portraits and a 35mm for walk around, both used but in mint condition.
I'm still keeping hold of my Olympus m4/3 system for the time being because of the excellent quality of the lenses, I'm still not that happy with the sensor especially in certain lighting conditions.