For #1 you wanna try Magic wormhole. Maybe it's less user-friendly than you need it to be, but it works and there are lots of implementations for different owes (don't know about iOS though).
That is kind of the problem though. There are many solutions, all with their own pros and cons. But after all these years no universal standard has managed to appear.
I used to use Pushbullet. Haven't really needed it in a long time since discord came on the scene really. But it did the job really well and was super easy to use.
they both do private file sharing, but their working principle is inherently different: wormhole, localsend, pairdrop etc. send a file once, whereas syncthing aims to sync a folder on 2 or more devices bidirectionally