I also use Posteo, one thing to note though is that Posteo doesn’t (and probably won’t any time soon) support custom domains. If that doesn’t bother you, it’s a great choice.
The other alternative I found during my research, which doesn’t have that limitation, is mailbox.org.
This could be a long shot but if it’s a Samsung Galaxy Tab S, the battery connector solder joints kinda die after a while, which can either be fixed by reflowing them or by applying pressure (I have a tablet with that issue). You could try pressing on the battery connector and see if it works then.
Most mobile devices won’t work without batteries. The best way to work around this is to supply it with 4.2V (anything between 3V and 4.2V will do) over the battery connector.
I really like Pop!_OS, AFAIK it doesn’t have any telemetry. It’s basically a Ubuntu fork but without the stupid Ubuntu stuff, and they’re currently even working on their own Desktop Environment.