It’s great to read that you like finger drumming with the XpressPads technique so much!
I can imagine using a foot switch to change presets is possible via the midi standard, but I have never done that. So, unfortunately I cannot give you a recommendation how you could do this easily.
Another option I thought of after you mentioned that you don’t mind buying another pad controller is to buy one that lets you change preset banks by just pressing a dedicated button per bank. The MPD232 has 4 pad bank buttons per preset, but it isn’t cheap. The MPD218 is inexpensive, has great pads, and lets you toggle through 3 different preset banks with one button.
I didn’t play finger drumming with mobile apps so far, but I heared about iMPC and iMaschine. However, I am not so convinced that the performance limitations that mobile devices have will yield good or realistic drum sound results.
I hope you’ll enjoy Magic Tone Network which is coming soon. I personally have lots of ways I can use it in my music productions.