I went mirrorless and got an adapter for my existing EF lenses
Edit - If you have a look through my post history (not my comment history) you’ll see lots of bird photos. Anything with a 2023 or 2024 date on it was generally taken with my old EF 70-300mm lens using an adapter ring to mount it to a Canon r50.
Have you run into any caveats while using the adapter? I’ve heard comments about adapters sometimes causing issues with EXIF data, but I could easily see that being caused by cheaper adapters.
You can buy cheap dumb adapters that are just metal rings and transmit no data or more expensive adapters which transmit data and allow autofocus. Buy cheap adapters for any old lenses you want to use and expensive ones when you value the lense tech.
I’ve got a D750 and an XT100. Given the smaller size and the cheaper old lenses I can run the XT has gotten more use in the past few years.
Since you mentioned the XT100, what do you think about the lack of the large right hand grip versus your D750? I like the retro aesthetic, but I’ve got larger hands and I’m unsure of how comfortable it would be long term.
My hands are average so I haven’t had an issue. I bought used and it came with a thumb grip. Also I keep a leather shell case on it, feeds into the retro look and adds some bulk. At no point have I found it uncomfortable and no one else who has picked it up has complained given what I’ve added to it.