@ST The autofocus is crazy fast on the new 300mm.
The a9III pre-capture is great for birds, at least for me. Without it I almost always missed the bird taking off shot, I just didn't respond fast enough.
An advantage the a1 has over the a9III is the resolution, if you anticipate heavy cropping its nice to have those extra pixels.
On the other hand, with pre-capture on the a9III I can come in closer than I could previously, since I can get that initial moment instead of needing a wider frame for safety.
It might be worth trying to rent it and see what you think?
Tony