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.
Here is a possible case study for you. https://OpenRouter.ai.
They are doing what you think of doing, for the various LLM APIs. Three reasons why they offer added value: