

KeePassXC you would put the sync-file itself into syncthing or something, and then KPXC would resolve changes between the sync file back to the main vault. I don’t use this method directly so I might be incorrect on the details, but it is possible to setup in a device to device manner.
You keep saying external server for syncthing, but again: syncthing does direct data transfers, encrypted end to end, between devices. It does not use cloud hosting or servers. It has the equivalent of a 90s FPS matchmaking lobby, so you can find your own devices latest IP.
You register the devices with each other with their generated ID codes. Then you ask the matchmaking server when it last saw that alias. It gives you the last IP that checked in with that unique alias. It then contacts that OP, and performs a handshake. If it passes, your two devices can now sync directly. The matchmaking relay has 0 data of yours, and 0 ability to associate your unique ID with a name, hardware, or anything other than a last seen IP. When on the same LAN, devices don’t even query the matchmaking relay if you don’t want. It’s totally offline.
If you elect to, you can allow relays to let you tunnel of you have NAT issues, and your end to end encrypted data can be synced through a relay. In those cases then yes, you are extending a bare minimum trust, and you fully encrypted data would temporarily pass on the relay’s RAM. If this makes you paranoid, you can easily add a password to the sync folder itself, encrypting it unless another user inputs the password on the other end. Adding another layer if you wanted.
I just get nothing from Bitwarden that syncthing and KeePass don’t offer more easily. Syncthing works for tons of devices and other purposes as well, preventing to host a password sharing only tool, and just letting you use a direvy device to device sync tool. I don’t know how or why you would have vault conflicts, but it really does sound like something fixable. Running this for years and I’ve never run into it.
This is the way.