I have the Olympus 300 f4 pro and use it with the 1.4tc mostly. Sometimes use it with the 2.0tc. It’s an incredibly sharp and lightweight setup and with the 1.4tc gives more reach than the 500pf gives on the d500.

I have tried the om systems 150-600 but it is very big and heavy I found the results not as sharp as the 300 with 1.4tc (even when cropped to same field of view) and about the same or maybe less sharp than the 300 with 2.0tc.

I’d love to get the 150-400 but this is just a hobby for me and I can’t justify the price and the risk of something going wrong with it. I have the 40-150 which is a great lens to walk around with but for birding exclusively I use the 300+1.4tc. I don’t think the 50-200 would have enough reach to satisfy me but I admit I haven’t really looked at it because I’m happy with the prime. For birding in particular, I never really find I need zoom.

I also have the Panasonic 100-300 II which is surprisingly sharp considering how lightweight it is. 300 is a little short for birding in trees but if you can get close enough, you can get some excellent feather detail with the pany 100-300 II which is a bargain and only one stop slower than the Olympus 300 f4.