MIDI-Export Settings

I'm a choir conductor and write most of the arrangements myself.
SO I set up an Arrangement Sop, Alto, Ten. Bass and musescore sets the sound to choir.

I then change the sounds back to piano in the mixer, in order to hear the correct rhythm etc. The choir sound gets a bit blurry.

Before switching to musescore I usually sent out MIDI-Files to my choir members, where I set their part a bit louder and the other parts softer in the background, so they can rehearse their parts on their own.

Now on musescore, I tried to do the same. I changed all sounds to Piano in the mixer und dragged the volume to the level needed. (all parts on -22, but one stays on 0) When I now export the MIDI-File, I receive a MIDI that plays choir sounds again and they are all set in the same volume. Even if I reimport it to musescore itself.

How can I set the volume and instrument settings to maintain in the file?
(I am aware that MIDI is a controller file not an audio file. But the controls should save the volume (in a numeric value) and the sound anyways, right?

Thanks for your help in advance