peacockmedia.software/kalimba I recently demonstrated using MIDIKalimba with an MT32-Pi for sound, which works beautifully. Here is another option, possibly more portable and convenient.
As much as I love the idea of being a witch, it's all illusion. I may have acted that part about three times and you only see the take where it went smoothly. Of course no-one ever gets that right first time 😆
As simple as this is, I too never really thought about using a phone for soft synth. I have used an iPad though (which is more or less the same thing).
I'd never use it in preference to a desktop, but it's a nice-to-have when you're away from the office because it's what you already have in your pocket. It has more possibilities than the MT32Pi and I think the thing I was most pleased about is that it happily powers the device from its own battery, eliminating the need for a separate powerbank.
Sounds very good, there are some moog instruments for iPhone, could be fun. I use koala on Android which is very good. Coincidentally I was listening to tuxedo moon while I waited for a pizza, I saw your upload and clicked on it straight away, it sounded like a continuation of the tm track.
I haven't yet explored what else is available. Am I right that Android also just lets you plug in and use a USB MIDI device with no hassle? Listening to Tuxedo Moon now.