BIF with R5 and RF 100-500mm compared to A9II and FE 200-600 G

I own or have owned the A9, A9II, A1 and R5.  I own the 200-600 and owned the 100-500.

They are both excellent systems/combinations.  It is a hard choice between them.

You are correct in that the resolution of the R5 will help make up for the loss of 100mm.  However, one thing to keep in mind is you are comparing 24MP worth of nice big FF pixels on the A9 to 24MP of smaller pixels after cropping the R5.  I believe that the 24MP FF A9 image is nicer compared to the 24MP cropped R5 image. I don't know how much that really matters but may matter in lower light, higher ISO situations.

You mention a difference of 2/3 stop but at the long end it is only a difference of 1/3 stop (6.3 v 7.1) so not too significant.

I think you are aware of the main differnces between the two lenses.  One is larger and heavier.  The other can focus closer for semi-macro and is smaller and lighter.  The 200-600 has much smoother and shorter zoom throw.  If you do any shooting where you zoom back to acquire and then quickly zoom in to 600 to shoot the 200-600 is so much better.  The 100-500 is stiff and a long throw.  I found the tension ring to be too loose at the tight end and not loose enough at the loose end.  I have the same opinion with all the 100-400 type lenses I've owned including the Sony 100-400 and Canon 100-400II.

As far as AF goes, the big attraction of the R5 is the Bird Eye-AF.  However, I found that to be most advantageous for perched or floating birds.  For BIF it does activate sometimes but I didn't find it any better than using my favorite Zone or Wide modes (non-tracking) for BIF on the A9.

My feeling is that the A9/A1 still has better AF than the R5 for very fast reactions to a fleeting, fast BIF that comes out of nowhere.  I found the A9 to be a bit more sticky on the BIF if it started to track it properly.  I found the R5 to be a little more likely to jump off the bird and grab distracting water or backgrounds.  That said, I think the R5 is excellent AF for BIF and will get you a high keeper rate.

For non-BIF I found the R5 superior because of the Eye-AF.  Of course now that I have the A1 I have Eye-AF that is almost on par with the R5.  I find the R5 picks up eyes with BIF more often than the A1.  The A1 requires very smooth panning to see eye-AF show up for BIF...but I've had it show up even for swallows IF so it is possible.  The R5 recognizes body and head shapes early on and when the bird is in weird poses.  The A1 seems to require the bird to be in a more normal pose and really doesn't have body/head detect.  I find that once the bird is in a pose that you'd actually want to push the shutter, both A1 and R5 are equally effective for Eye-AF working.

I decided to sell my R5/100-500 kit to pay for the A1.  This was mostly because I had a better investment in Sony lenses and liked the 200-600 more than the 100-500.  So I sold my entire RF kit, bought the A1 and now shoot it with the 200-600 and 600GM.  I repurchased a Sony 100-400 which I had sold when I bought the RF 100-500.  If I had still owned some of my larger EF Canon superteles I might have gone the other way.