modern versions of Seedvault work pretty well, especially with A15, there’s D2D mode now (so it backs up more data for most apps), integrity checking, deduplication of app backup data so the fact it backs up more isn’t as much of an issue and a bunch more
IMO it works pretty well, I use CalyxOS so it lands those changes pretty quickly, I’m still on A14 though, but I don’t have much to complain tbh.
If you’re using an older/unofficial version of LOS, your exp might vary
to me, the only apps that don’t get backed up are Signal, WhatsApp (which I don’t even use) and Element. For the first two, I just add their independent backup folders and for Element, well, that’s online.
It sounds like the solution is to search for “How to migrate “AppName” to new phone” instead of “how to migrate fdroid apps to new phone?”
For example, I am not looking forward to recreating my Revolution IRC config on a new phone, so I would really like to find a way to migrate it. But a search reveals no results. So it looks like I am forced to recreate my configs on the new phone.