Your idea of combining RAK1920+RAK1910+RAK18001 is fine. You just need to take note that RAK1910 already uses UART so it is recommended to use external sensors peripherals that doesn’t use UART. I2C is best.
For LoRaWAN button, please have a look on this two detailed tutorial:
Thanks for your response. I am considering using instead the Grove 12 Key Capacitive I2C Touch Sensor v2 (MPR121) connected to the Rak1920. Will this touch sensor be able to work with the other sensor without any incompatibility?