sh1209
At B&H the Sony A9iii is $6000. An OM-1 mk 2, $1900 on Amazon, plus an Olympus 300f4, $2800 at Adorama, is still $1500 cheaper even if you already have a Sony 600f4 lens. Both shoot high f/s in pre-capture in RAW. This is why I recommend the OM Systems rig over the Sony A9iii.
However, choosing between 50mp @ 30 f/s and 60/120 f/s at 24mp is a difficult choice. You really need both plus a large buffer.
In addition, if you like the subject large in the frame as I do, I find that I need a zoom to properly frame the image in order to provide the space in the frame where the BIF will be. Typically, when I shoot a prime in other than a controlled situation I either don't have enough reach and the resultant image requires excessive cropping, or I am too close, and the wings get clipped on the shot I really want.
My current solution, believe it or not, is an OM-1 mk 2 plus a 100-400 and a 1.4 TC @ 120 f/s. This is a $3500 solution. At 1100mm+ (ff equ) I can keep the crop reasonable, and I get lots of choices. (Obviously I need lots of light.)
The better and more expensive solution is the $7500 Olympus 150-400 which seems to outperform any zoom and is equivalent to my prime according to reports, but I haven't spent the money yet.