My docker containers are all configured via docker compose so I just tar the .yml files and the outside data volumes and backup that to an external drive.
For configs living in /etc you can also backup all of them but I guess its harder to remember what you modified and where so this is why you document your setup step by step.
Something nice and easy I use for personal documentations is mdbooks.
Idk what's offsite to you but if it's a controlled place for you (like from a friend or family member) you could simply bring your device one day and do the first copy there.
Termux has nginx, postgres, python and plenty of stuff compiled to ARM so I bet you can. You would have to be wary of non standard ports unless you have root access and make sure android does not kill or puts to sleep termux by adding exceptions to the app.
I remember running a few low traffic Mastodon bots in a S3 Mini years ago and it was decent.
You only receive content updates for the communities your local users are subscribed.
That being said federation has been struggling significantly since this past reddit exodus and there's time where outages of any kind might have made you lose federated content. I don't think is possible to "catch-up" all of what you have lost.
You can however per example search a port url of a remote instance to force it federate.
I guess it might increase the load slightly but I think individual instances actually do more harm when they go down and disappear because of the timeouts.
I run my own individual lemmy instance where I'm by myself because I don't want the extra legal burden of random users signing up here but I also host two communities with around 100 subscribers and not duplicated on other servers so I guess it works to actually contribute to decentralization of the federation.
UP