I have just red your last link:
https://mclarenlabs.com/blog/2019/01/korg-microkey-air-37-bluetooth-midi-keyboard-with-raspberry-pi/
In this example, a ble midi device is connected to a raspberry with bluez-4.8 and all works great. I think (I am not sure) that with midi ble connections there are midi periferical and midi host.
In the link example, the rapsberry is working as “host” and a periferical is connected. I understand that bluez supports midi connections when a midi ble periferical is connected.
In my case, I would like to connect two rasp, or an android to my rasp… So I need enable a raspberry like a periferical ble midi-device. As I undertand it, I need btmidi-server in order to convert my rasp in a ble midi device… but btmidi-server is not available in bluez 5.50 (I dont know why).
:?