My preference is simple: a one-time paid purchase, either in-app or up-front (allowing for at least a preview).
- I’d be perfectly happy with paying $25~60 for the app, if my security could be assured. A closed-source wrapper doesn’t give me that feeling. Open-source reproducible builds do.
- Any subscriptions means an app is immediately deleted for me.
- Any ads mean an app is likely deleted for me, as I’d have to assume tracking is involved.
(as an aside, opening your source wouldn’t cause most people to bypass payment, as it is a royal pain to keep apps authorized on a factory iPhone without an Apple dev account. I’d rather pay for the compiled and supported product.)
Hope this anecdotal feedback helps 
EDIT: to be clear on my tone here, I’m happy and excited for this! Not a fan of the iSyncthing name, and I would be far more comfortable with a fully-open-source project, but if the open-source hurdle were fixed I think you’d be pleasantly surprised on how many iOS users would be willing to pay handsomly to bring their beloved phones into the secure Syncthing ecosystem. I’m certainly one of them.