Iwouldnt sweat it and start messing with your feed to much some of my elite cuts that are finiky on nutes and enviroment show this sometimes, its in the genetics the flowers will turn out fine my friend.
I highly disagree about moving forward on an breeding project with a PM infested plant. I believe it may be or act systemic. I understand science doesn't support my position at this point but 2 decades working with the plant proves otherwise.
If you are blessed with the space breeding for Pm...
What are you looking for? im assuming your purple is some kind of purple kush. if you have a scope you can harvest based on percentage of amber or if not early harvest will generally be a more racy high, and later will be more sedative with higher CBN levels.
alot of De brands produce low pro versions now as well. I run classic De gavitas with 9ft ceiling and large plants without an issue. your space seems perfect for a 4x5 pattern cheers