You can use Seedvault backup for apps that allow backup. Integrated into LineageOS. Just make sure to copy the backup to your PC or select a micro SD card as storage location directly.So there is no way to decrypt either prior to upgrading (disable FDE)? A clean install would mean about two days work to get everything setup correctly again, something I want to avoid at all cost. I have certain apps that require an entire process of identifying yourself to get them working again, a lot of hassle. Not even sure if it's worth upgrading then as 19.1 still works good, but I'd like to stay up to date security wise and 19.1 is no longer getting security updates either.
For apps that don't allow backup you can use NeoBackup. It will probably not work with all apps. But in the worst case, any app you might use will have some procedure in place to allow you to migrate to a new phone / a fresh install.
You will have to backup internal storage yourself (photos etc.), if you don't sync to google cloud or something.