The ESP32 is certainly an interesting (and powerful) platform, and I considered using ESP32 in the beginning, especially for the possible MIDI over Bluetooth. The reason I went for the pro micro was mainly cost, size and that it has enough I/Os and performance.
However, the S2 has a lot of interesting features for future devices:
· USB host capabilities
· Lots of performance (ram, flash and cpu)
· I2S & 2x DACs
· WiFi (configuration over html)
I see no disadvantage to rebase the code, but for a simple keyboard, I see no real benefit either ;)