A TWRP backup would be useless if not for your model only.... You can't restore a backup of a different Model on a different one even thou it's the Same Make.. So I guess the only options are listed above... Titanium backup is really Good for backing up of apps and data...
yes, TWRP is built OS-version specific
the reason is changes in android versions may differ in
file system type
most (if not all) updates are backwards compatible. that means the TWRP is ready for android 9 but you can still run android 8 on your device