[murilopolese718523034] suggested WiFi + Websockets + ESP32 running Micropython.  

Websockets are great b/c you could code the app on the phone side as a webpage, which makes it more portable and relatively easy to hack together if you know HTML / JS.  It's also (relatively?) low latency, at least compared to HTTP.  Which is faster, Bluetooth/Serial or WiFi/Websockets?

I also like this b/c the hardware side is pretty easy to handle.  Just an ESP32 and you're done.  Let its HW peripherals handle the SPI or I2C or ADC or whatever.

What's missing is a protocol/API.  

Micropython here or there, this is a promising approach to tacking fairly high quality GPIO onto a phone.

Pros: cheap, readily available, super-flexible hardware

Cons: battery drain from WiFi, flexible -- protocol implementation needed

This is the angle I'm going to chase down.  I've got some ESP32 breakouts just asking to be used...