What does a backup of all user-accessible data contain, and how to restore to fresh Android?

Using /e/OS Setup
2

There isn’t. Android is not nicely organised Linux.

I think you need rather a strategy to back up most important things first, then watch carefully what other options offer. Seedvault [1] seems a good bet. Reading carefully what is can do is quite explanatory of why Android is so difficult to do “everything”.

[1] Settings > System > Backup