After testing out a few more times, what I am noticing is that small birds need a really high shutter speed in flight. I originally used 1/1600 minimum, then went up to 1/2400, still seeing plenty of blur... So I might have to have two custom modes, one for big slow birds (herons, hawks etc.) and one for small fast birds, maybe 1/3200.
I like the fact that in A mode, when light isn't good enough, first it starts increasing ISO toward the ISO-A max (which I have set to 12800), then when it hits the ISO-A max, it starts dropping the shutter speed below my "mimum shutter speed" setting. That's pretty much what I would be doing anyway if I were using S mode.
The only issue really is that the minimum S/S needed, varies quite a bit depending on the subject.
Currently I have a third custom mode for low-light situations, which is S priority, where I set ISO-A max to 102400, so I can more directly deal with the compromise between S/S and ISO.
Another thing I'm noticing is focus peaking seems easier to work with in the OM-1.2, than in my older model. So it is pretty easy to go right to manual focus when the bird AI fails, for birds in the shrubbery.
Here is a nice video in which Espen Helland talks about the tradeoffs of back button focus vs. shutter half-press focus.
In the latter half he discusses backup solutions when bird AI doesn't work which I found really helpful; basically switch to small target for more direct AF, then switch to manual focus. This is intuitive for me and I'll be practicing this. I suppose I already sort of had this thought, but it's nice to see an expert confirm and clarify it.
One thing about BBF is that for the "burst" type photography, one is already using the shutter continuously. So having to maintain C-AF by also holding down the AF-ON button seems a little excessive. So I'm not sure, there may be situations where it's better to use the shutter half-press. This is one of the things I'll puzzling out. Another issue is how to smoothly switch between Pro Capture and regular C-AF in a moment-by-moment situation as the birds are flitting about.