I've a single Pihole instance running on a RPi 4 and have experienced not a single instance of any of the 3 probs you mention. Except, of course, the very few minutes it takes for a reboot which I can schedule and am aware when it's happening...

Yeah, I believe it can vary depending on how you host it.

In my experience whenever I brought down the PiHole instance (Docker Compose) I would lose all internet access, which is expected since I’m essentially taking away my devices one and only library, so to mitigate this I spun up PiHole on another device and set that as my secondary (backup) DNS resolver.

This way I can take a container down, update it and all without losing resolution to the internet.