Thank you for asking for community feed back .
I agree with tap and hold
a debuff of up to 25% to helminth would seem fair to keep it from skyrocketing at THE helminth ability to use.
and as a suggestion to consider having it to work with a similar mechanic to guaruda's passive and citrine's 2nd ability and calibans passive and the mod adaptation.
Damage - kill enemies to build up damage buff and gradually decline when not killing.
DR - take damage to build up dmg reduction and gradually decline when not taking damage.