If you completely lose your password to your vault there is nothing you can do, simple as that. Don’t lose it.
Unfortunately, as mentioned in the post, there are some ways to lose access to your password that are out of your control. Furthermore, the more places you store your password the less secure it is. It would be a lot easier to be able to authenticate with multiple authentication methods individually, than to rely on having access to all of them at once. That’s the problem I’m trying to address here.
Cloud-based sync is incredibly easy with self-hosted cloud, as pointed out by the KeePassXC FAQ. Self-hosted cloud is effectively a local solution.
It is still subject to the issues listed in the 3-2-1 rule, however the goal of self hosting itself conflicts with that rule (since the rule dictates the use of off-site cloud storage). I will note, it does somewhat solve the issue of keeping database backups, as any device pulling from the local cloud server effectively becomes a backup of your database.