I don’t use Syncthing, but a part of that is because I did not think it was even available in the App Store?
In any case, there is no way to get around the iOS requirement that your Logseq graph be stored in the Logseq icon folder. Apple sandboxes apps as a security measure, and I don’t think there’s any way around it without voiding your warranty.
In general, the preferred method is syncing with Git, avoiding any cloud service.
Most people use the Git Sync 101 guide which advises iOS users to get Working Copy, but if $20 isn’t worth it (I balked for days, then caved after remembering worse spends) there is also this free, if more technical, git method for iOS using a-shell.