I have the dimmer for the Inventronics that I can get a pic of if you think that will help wrt dimming...
So basically a variable voltage output from the reef-pi and the lights take care of the rest?
Cant hurt to get that picture as well.
Here is the manual for that driver.
https://www.inventronics-co.com/wp-content/uploads/2017/12/DS-EUC-052SxxxDTST_Rev.L.pdf