Thank you all for your generous responses so far. I do have a good sense of the technical trade-offs involved (which is why I decided to go full frame); in addition, my system wouldn't be changing much in size, since my current 200-600mm lens is designed for full-frame bodies and works with the A1.
Instead, I'm mainly curious about the experience of transitioning, which DRwyoming kindly addressed above. What is it like to feel farther away from your subject, especially when photographing very small birds? Did you have to go through an adaptation/frustration phase? If yes, how did you work your way out of it? I've been thinking of getting a 1.4 teleconverter to compensate, but I've heard that unless you attach it to a big prime (which I can't afford), they will give you similar results as cropping the image in post. I wonder if that has been your experience with that particular configuration (A1 + 200-600 mm).
If you use say a 60mp full frame with the correct shutter speed, the correct exposure, with a floated iso, you should be able to have significant gains with crop ability that surpasses a cropped sensor performance especially in iso.
The one thing that is important is focus permanence, i mean first comes light then focus, then speed, then exposure, these are all more challenging for a cropped sensor if conditions become challenging................
The 6600 is a really great camera, for me personally its a brilliant camera for traveling and holidays.
Your Instagram shots are very nice record shots and represent a diverse variety of interesting different birds very enjoyable to see.
For myself the move to full frame higher resolution cameras (even bargain priced used one's ) opened so many opertuinaties and really transformed my photography immensely.
If i was like my ornithologist friend looking at just birds on a stick record shots in ideal light then i guess any camera is fine.
Your enjoyment is what matters, please yourself first second and third...........LOL
As Northern Focus pointed out, at a touch of a button you can switch from full frame to cropped sensor.........
What i do at times with my D850 or Z9, is i use the 200-500 with a 1.4TC III in cropped mode therfore using the center of the lens where the performance often is best, this gives amazing results especially when covering surfing events that i often do where reach and or the perception of reach matters, having a high resolution full frame camera really helps with greater tolerance and results in post.
Decision time best of luck, remember 80 - 90% of what you get comes from you.
Light is your greatest asset.