First, skip this project entirely and buy a direct midi to Bluetooth adapter. https://www.sweetwater.com/store/detail/MDBT01--yamaha-md-bt...
Second, use a MIDI to USB cable, https://www.sweetwater.com/store/detail/Uno--m-audio-uno , and then plug into your Raspberry PI. You can also buy USB MIDI to Bluetooth adapter for about 50$.
Finally, you could buy a MIDI hat for your Raspberry PI. I don't think this will work out of the box, but I can't imagine it would be too hard.