Mills gives a one time bonus of 5%, which I admit kinda sucks... Changing it to a recurring means you have to add a new trigger (so might not be savegame compatible).
Heres a simple trigger that I made from scratch that you can add that will keep applying the bonus:
720.000000 3 6 3 1224979098644774912 648518346341351501 648518346341351591 541 3 1224979098644774912 131 1 1 3 936748722493063465 1224979098644774912 5
What this does is: Every
720 hours (30 days) it runs a check through all the villages. For each village with a mill, it triggers a script that increases prosperity by
5%.
What you do is open up simple_triggers.txt and paste that whole thing in at the very bottom, then go up to the very top and increase the very first number in the file (84, on line 2) by 1 (so 85 unless you've added other triggers). That way it will read the trigger. Again, I don't know if this is savegame compatible - I don't have a save with a mill built to see if it is or not. Also note that while this trigger *should* work, im only human. I'll be using it myself, though - so I do trust it