Your D800 remains a very capable body. The three advances since them are quieter mechanical shutters (not all brands embrace them equally), pretty solid electronic shutters (so you can pass on the loud mechanical shutter when needed), and face/eye/subject recognition (makes getting the shot a lot easier). There have been some advances on the ISO noise front too, but these have been slow and steady IMO.
If you frequent dimly lit places and your kids are constantly on the move, it’s hard to beat a MLIC with a fast prime. I do occasionally miss my speedlight, diffusers, and umbrella for the effects it could create but I get by in dim light pretty well without them.
Coming from a D40, D5300, and getting back into kids with a J5 it’s kind of funny to find myself standing by a A7III, but it’s a great value these days and delivers more consistently eye/face in focus photos than a Z6II.