I put my database within the path that’s mounted to my nextcloud container. KeePassium on iOS lets you connect to WebDAV which is one way to have Nextcloud host it. It’s good about letting you access the database offline if you lost connectivity.

3:2:1 backup can still happen via whatever method you use for all the rest of your files. So far this is working fine (albeit a bit slow at times) on an RPI. Remote access via Wireguard VPN.