Self hosting means you have outside your phone your real vault and the phone is just connecting to it to refresh its local data.
I’ve setup my vaulwarden in my local network kit’s the local bitwarden server i use), my phone, tablet or simple webbrowser can connect to it when i’m home via the classic bitwarden (with self hosting parameters).
If i travel, i have just to start my openVpn session and connect to my home but it’s only needed if I want to update something (the encrypted cache it’s enough for consulation). If I have nothing to change, no need to have a vpn. I just use the cached data.
If my phone is stolen the data are safe (cache is encrypted, source is not on the phone). I revoke the vpn access by precaution and move one. No sms scenario needed here.
You only need to have a backup phone or computer to setup your new access on the new phone.
Edit: of course my vpn connection is protected by a passphrase so nobody can connect to my home network without me around. And the bitwarden app is also protected of course.