Hi, I made a type design app for iPad that supports exporting Variable Fonts (in CFF2 format).
Some of its features include:
The app is still relatively simple and doesn't yet support many functionalities such as hinting and color font. Also it might crash occasionally.
If you are interested just search for "Letterforms - Font Maker" in the App Store.
Any questions are welcome, especially regarding the setup of the Variable Fonts technicalities!