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.
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.