Something like Syncthing, perhaps?

Anyone know of any mobile apps that have done this and bundled their own fork of syncthing under the hood for syncing?

Practically not really needed for a person going out of their way to setup syncthing, you can just sync the underlying folder, I do this with logseq, their syncing subscription is paid, I just sync the underlying logseq graph and markdown syntax. It's seamless and rarely disturbs me, works well in background, although android seemingly doesn't respect my background preferences, and clears it out of my ram when I inevitably hit the clear button, but that's soluble by simply rebooting once in a while.

right now its in webrtc

There are a lot of valid answers to this! One is to use your platform's provided one, like OneDrive or iCloud. Another is to integrate with some other sync platform. Dropbox is a popular target for this. Peer-to-peer is another, although that obviously also come with limitations. Finally, bring-your-own-sync is a popular choice amongst open-source apps, where you provide a self-hostable sync server.

Note that this thread is full of people claiming that using SQLite with iCloud sync is evidence of some conspiracy theory or other!

There's a git plugin.

Check out Aardvark (renamed to reflection) it's a collaborative note-taking app from the GNOME folks. I think the idea isn't to completely remove cloud infrastructure, but to at least make it optional and/or provide alternatives. For example, this note app works via P2P. blogs.gnome.org/tbernard/2025/06/30/aardvark-summer-2025-update/

You can use FTP and SVN.

Both of those require a server

Syncthing

The benefit of local-first means you’re not incentivized to sell your cloud offering, so you can just give options. Sync with iCloud, Google drive, OneDrive, Dropbox, Mega, SMB, SFTP, FTP, whatever you feel like adding support for. And since local-first usually means having some kind of sane file format, you can let “advanced” users manage their own files and synchronization like people have been doing for the last 50 years.

Ideally, you would use existing commodity infrastructure but we have found none of it is really super fit for our purposes. Failing that, we have been developing an approach to low-maintenance reusable infrastructure. For now, I would advise running your own but positioning yourself to take advantage of commodity systems as they emerge.

For Joplin I use WebDav from the 10gb of free file storage that comes with Fastmail. So I have easy sync with multiple platforms and form factors, and even substantial notes make little dent in the allowance.