Simple ? - Can I specify - somehow - an axis with a M220? I think that answer is no. It just seems I would prefer Z not to be super slowwww when I put it at 50% feedrate. Z is slow enough at 100%. My Z hops are slow up and down and they have default Accell and Jerks.
And for some reason OP says it can't read feed/flowrates. It isn't a firmware limitation.
Setting the feedrate affects all axes. It is a multiplier to all speeds specified in the gcode. You would have to change the Z speed in the slicer you use if you only want to affect the Z axis.
There are only commands to set the feedrate, none to read them (not until recently anyway). That is why for OctoPrint the feedrate and flowrate are "write only". Yes, this is a firmware limitation.