This weapon is already too weak to begin with and doesn't need further nerfs. Several people have already expressed doubts about its usefulness. I think you have greatly overestimated how strong this is, and should be looking for ways to make it stronger. (Hopefully without deviating from Hollow Knight too much.)
Alright, I’ll remove that. So, here is my idea for the reworked Dreamer buff:
Grants no mana anymore, but dramatically increases mana regen rate, allowing it to regen mana even while attacking (albeit with very much lowered regen rates)
15% increased damage for all classes
12% crit chance for all classes, including Summoner.
3 HP\s and skips a lot of the life regen ramping up.
The next attack you land creates one of four spectral weapons that strike your foes for 100% of your weapon’s damage:
(This is obviously not going off of the original content, but I think this would be cool to see)
Skofnung, the sword: Appears behind you and swings forward, cleaving through all foes in front of you
Pinaka, the bow: Fires an infinitely piercing arrow that shreds foes with a powerful debuff
Thyrsus, the staff: Creates a quickly expanding web of vines, which grow explosive grapes.
Erymanthian, the boar: Rears up, and charges at foes three times, before a spectral spear impales it’s side.