Would anybody have any tip about it? I still can't find the Arduino USB2 Serial Micro to buy and I thinking of changing my hardwares. The options would be finding another USB conversor which has an AVR chip or finding a board which has a second AVR chip used as the USB controller. This second option is not so good because the idea is to have a compact device after the mounting.
Replace the Pro Mini and USB serial with an Arduino Micro. The Micro uses a 32u4 microcontroller so it has a native USB port. There is a library to send MIDI over the native USB port. No need for hairless.
There are many other boards with the 32u4 chip such as the Pro Micro, Leonardo, ItsyBitsy 32u4, and Feather 32u4. Any one can be used instead of the Arduino Micro.
gdsports:
Replace the Pro Mini and USB serial with an Arduino Micro. The Micro uses a 32u4 microcontroller so it has a native USB port. There is a library to send MIDI over the native USB port. No need for hairless.
There are many other boards with the 32u4 chip such as the Pro Micro, Leonardo, ItsyBitsy 32u4, and Feather 32u4. Any one can be used instead of the Arduino Micro.
Thanks for your answer, gdsports! I wanted to ask something more, then. In the repository of the project they provide the code to upload in the Pro Mini, which uses only the MIDI library, not the MIDIUSB. It is here: Arduino/MrHeadTrackerBNO055Switchable · master · DIY / MrHeadTracker · GitLab. Using the 32u4 chip requires that I use specifically the MIDIUSB library without using the HIDUINO firmware or would I still be able to use the code in the link and make the step of reprogramming the MCU with HIDUINO firmware? I ask this because I wanted to follow their steps as I am completely new to arduino and would not have enough time to make/change the code until the date I have to finish the project. The head tracker is only the beggining of it all...
"HIDUINO takes advantage of Arduino boards where a second AVR chip is used as the USB controller, so it won't work with single chip variants (e.g. Leonardo), nor older boards that use an FTDI chip as USB controller (e.g. Duemilanove)."