iOS has Möbius Sync as a Syncthing client, but it is not free ( but only 5$ one time payment) if you want to sync folders from other apps like Syncthing. The option to sync other folders was introduced this year.

I am also using iOS with Möbius Sync and it is working quite well. The sync is not running all the time in the background due to iOS restrictions but it is running multiple times per day on my device.

The option to sync folders was only introduced this year. It costs about 5$ one time payment.

There will be many conversion tools for Joplin to other structures. So I guess the format is a non issue.