I would use the stock 10 and its twrp
I will later try (after exams) to make a stock/sodp independent twrp which then may work with earlier android versions as well (issue is the different dtbo partition which is different on different kernels and different android versions)
Maybe this bug report about backup and restore is interesting
https://github.com/TeamWin/Team-Win-Recovery-Project/issues/1542
I would anyway look in their bug tracker
Thanks for clarifying. I've been on this all day and no result. I will proceed to Stock 10
Edit: I have Android 10 installed with TWRP/Magisk and it is the same situation. It does not restore from TWRP. It defaults to Fastboot and no operating system. It's not just the older Android versions.
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