One thing you haven’t mentioned is unpairing and repairing the remote to the ATV. Maybe give that a try?

Haven’t seen any of those issues on my 1st gen 4K. Daft question but have you tried a factory reset? It’s the first thing Apple support will probably recommend. I’ve had AppleTV boxes since they were first launched though and never had an issue with them.

Did you unlink your previous phone from iCloud? Check in your settings to see if it’s still in the devices section and delete it if so.