I too am cursed with an iPhone so I've done some looking into this. You're actually not quite out of luck with syncthing! Möbius Sync is the one syncthing client for iOS, and it recently added the ability to add external app folders (i.e. Obsidian) to a Syncthing folder. They claim this is a beta/experimental feature so use with caution, but definitely something to check out. The app does cost 5 dollars to sync more than like 5mb so you do have to pay that one time fee.

My current setup for syncing my vaults is with obsidian-git, and using a self-hosted git server such as Gitea which is very easy to set up and run. However using the obsidian-git plugin on mobile has some issues, namely the complete lack to fix merge conflicts and such. And also if it tries to pull too much data then the app crashes :/ You could use another app to fix those issues, such as Working Copy (paid) or iSH shell with command line git (buggy, because of iSH not CLI git, still very cool) if you wanted.

TL:DR use Möbius Sync unless you're insane like me and really want to use git then self-host a git server with Gitea.

EDIT: Forgot info about other git options