Well, you can export from and to linkwarden and are far as I remember from the last time I tried it, you can export a generic json (which can’t be used to import to your browser). Not sure what changed since then, the New changelog says you can import omnivore exports now.
Well there is round sync, an rcx fork but that one of outdated too, now. Quiet sad as I really liked it.
Rsaf is different and does need a file manager as a helper. It’s still a great tool and I switched to it too from round sync. It’s well maintained.