I can not tell you how helpful your advice is — reading comparisons of options to specifically what I currently have. I saw a video linked from this forum reviewing the OM-1 (can’t locate it at the moment) He said it tracks birds well but then doesn’t hold the focus! That turned me off a bit to the OM-1. What do you guys think about that?
My estimations show:
I have an OM-1 and a Sony A1 and I never have to worry about focus on either camera. They are both excellent at keeping targets on moving birds.
OM-1 + 100-400 is $3300 and about 4lbs total
Unless the difference in weight is an issue I would think about the 300F4 plus TCs when needed. You can get the 300F4 used on fredmiranda and they go from $1200-$1700 USD (last one sold was $1200). I know the zoom is convenient but the 300F4 is a special lens (at least in my opinion) and worth a look.
Nikon D500 + 200-500 is $3600, 5-6 lbs
I had this combo and it's good but the 200-500 is only slightly better than your Tamron 150-600 (which I also had) depending on copy variation. You would get more pixels on target with the D500+200-500 (750mm equivalent vs 600mm) and faster focusing with the D500 so overall a nice upgrade however. You can also buy used and get it much cheaper than $3600. D500's are going for $800-$900 on fredmiranda and the 200-500 is going for ~$800 as well.
Canon R7 + 100-500 $4100 4.3lbs
Objective: I’m trying to improve keeper rate, sharpness, tracking of bird eyes, over my current rig of D700 + Tamron 160-600)
For me one of the big advantages of mirrorless is being able to shoot with electronic shutter (silent) 100% of the time. If that is important to you it really narrows down the choices, the cheapest being the OM-1.