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.
Technitium that I host myself. Works like a charm.
In case nginx does not work out for you, you can opt for more generic reverse proxies. frp is my favorite one: https://github.com/fatedier/frp
I have self-hosted my own emails many times. Up to having three SMTP servers with failsafe option at DNS.
It’s super nice, but I would never self-host SMTP again. It’s a nightmare. I had to email or open a ticket at most ISPs despite my clean IPs. Most ISPs simply blacklist all IPs unless they are major email providers already.
My advice is go for it but let SMTP be handled by who will deal with these frustrations. MXroute is a great choice and it’s cheap.
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/