What cloud service are you putting your images on? I would be shocked if google photos hasn’t already been doing this.
Also, you can restore an iPhone from what SyncThing can back up. Only an iTunes or equivalent iCloud device backup can. They also can’t back up things like MFA keys / seeds / IVs.
I’m glad ST exists and it’s useful for syncing photos for home use, but it is not a viable iCloud alternative, nor could one be made due to the private iOS API issue.
Imagine if your Tesla reported back to Tesla who was in your car and sent that to the authorities if they thought someone in your car was "illegal".