hoppy.network seems like a very expensive Wireguard provider ($8/month for 1TB@100mbit). For that purpose one can spend half that for a VPS with gigabit speeds, even a quarter that during promotions. That provides the same services plus whatever else you can fit to it. What am I missing that they provide?
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.