Good news Regoso,
Another modder pointed out to me how to make units region specific.
Within the db folder there is the file called "main_units". The second to last column is where the magic happens. Scroll down in the list and you will see that the Roman Auxiliary units all have their own unique values in this column dictating where they can be recruited. So rather than by province, it's by Roman auxiliary recruitment region. Which is... an interesting take on things, but it does make things easier as you don't have to pick the individual provinces. Notice that they have one for Hellas and another for Hellenic. Obviously, Hellas is restricted to that province alone whereas Hellenic encompasses that entire region including Asia and Magna Graecia.
I will play with this for a while working off your mod and my own reduced merc upkeep values to see what I can come up with