There is a self-hosted live sync plugin. It's rough around the edges but it mostly works and is actively maintained, if you are willing to self-host a sync server.

I say mostly works, because there are a lot of "gotchas" and the configuration and set up are a bit intimidating for the clients (the server is simple to host).

I used it for a while and it was fine, but I decided the cost of a coffee per month is worth not having to maintain it, and I switched to paying for their sync service.

However, there is also a git sync plugin that works really nicely. But it is not a real-time sync and it is not supported on mobile (officially). I mainly use that as a way to keep long running backups of my vaults in a self-hosted gitea instance (the default paid tier only keeps one month of history).