I still have K-9 Mail (POP3) and it was backupped and restored by Seedvault when I moved from CalyxOS to iodéOS.
Maybe you could try with K-9 Mail. As far as I understood, it is almost the same as Thunderbird but still kept for some nerds who like the old icon better 
Yes, it’s really nice to see the familiar homescreen launcher layout, complete with all the apps after a Seedvault restore. And it’s so easy to use.
In my experience, some app’s data could not be restored with any method, rooted or not. Typically banking apps, some health insurance apps, Signal, etc. Seedvault won’t even try (which is the best approach in this case), other methods try and fail (i.e. the app crashes after data restore and app data needs to be deleted for it to work again).
Then there’s a few more apps that Seedvault won’t backup/restore (and it gives a reason for that when you tap backup status which is typically “app doesn’t allow backup”). AndDiSa works for most of these. In my case the important ones were Thunderbird, Iodé Browser (restores everything, including settings, extensions and extension settings), Aurora store and FDroid. As Pete wrote there’s other ways to backup/restore the settings of these apps (but it takes a bit more time).
Well yes, Thunderbird syncing from desktop to mobile is fine, and yes I have used Firefox sync for years, etc. So I guess I wasn’t saying I didn’t know how to do it, and it wasn’t too painful, but just noting the apps that I had to re-configure.
What I found very helpful with Seedvault is that I could chose which apps to restore and which not. That was important when I moved from one custom ROM to another. It doesn’t matter, of course, if you just change devices and move from the same ROM to its’ mirror.
Is it possible with AndDiSa to purposefully restore only selected apps?
Short answer: Yes.
Longer answer:
The backup script creates a directory and populates it with two files per installed app: a `app-.tar.gz` and `data=.tar.gz`. The restore script takes the name of a directory as its argument, and attempts to restore all the `app` / `data` file pairs that are present in that directory. So to only restore specific apps and their data, create a directry with onl the files for those apps, and pass that as the argument to the restore script. And as the restore script wll tell you
WARNING: restoring random system apps is quite likely to make things worse unless you are copying between 2 identical devices. You probably want to
mv backupdir/app_{com.android,com.google}* /backup/location This will cause this script not to try and restore system app data
So you should in any case ‘prune’ the backed up files before attempting a restore. If you regularly want to only restore certain apps, you can of course automate the pruning with a script.
One fnal wrinkle: if restoring a backed-up app fails, the restore script exits: apps already restored will be present on the device, but the remainder will have to e restored by removing the failing app and data files, (and optionally any apps already rstored) from the directory, and restarting the restore.
Thank you for taking your time for the long answer, Pete! Really appreciate it and it might well be helpful for other readers.
That is where you lost me. I am an ordinary user and a n00b. In fact, I am not really sure how I even managed to flash my smartphone in the first place, knowing nothing about commands and adb and all this.
So my bottom line is: I will stick with Seedvault where I can just select apps on my display, without scripts or other wizardy stuff 
I agree. Backup and restore must be easy.
An important point for SeedVault backup:
“Backup my data” (experimental)
didn’t work for me. (At least not when restoring the backup to another device. I didn’t try the same device.)
Always back up your data in a different way and not with SeedVault.
My experience of backing up SeedVault in Nextcloud was not very good either. Aborts again and again …
Backups to USB are not practical for me. Usually don’t do that often enough. It has to be done automatically.
Therefore, I use the following backup strategy: