How do apply a 'lessened' potion effect? Also, how do i set a duration for it?

I'm making this mod that involves tons of potion making and brewing, and i was wondering how i could essentially 'dillute' a potion effect. Is there a way to make regen go slower, speed also slower, and strength weaker than all LVL 1 potions?

EDIT <ADDITIONAL QUESTION> : how can I get rid of all the current potion ITEMS and their obtainability in survival? would make my mod more of an 'overhaul' to the in game potion system.