Your requirement for a mobile iOS app makes it harder so I’ll go non-free software with my suggestion: Tap Forms. Offline first, iCloud sync, macOS and iOS apps. But no Android or Windows apps.
If you wanna keep it self-hosted, these services need an internet connection anyway, even Airtable. Just go with a Web based one that has good mobile layout.
If you have an offsite copy of your files (and not in a sync service like Dropbox) you are already in a better position than most.
Restoring from offsite takes time, even with Backblaze’s option of shipping a hard disk. You may also have data corruption troubles, companies may close all of sudden. It’s just not as convenient as local copies.
A further copy that is locally available is simply a better strategy. Adding more copies after these two is not a bad idea but you start getting hit by the law of diminishing returns.
You can actually read more about the 3-2-1 rule in a Backblaze post: https://www.backblaze.com/blog/the-3-2-1-backup-strategy/
Technitium that I host myself. Works like a charm.
There are some managed tunnel alternatives but they are not cheap. At this point it’s cheaper to rent a $20/year VPS and install one of these: https://github.com/anderspitman/awesome-tunneling
I personally love frp
Edit: I have never tried it, but there’s also Tailscale Funnel https://tailscale.com/kb/1223/tailscale-funnel/