This has been said a lot but a Tap/Hold would be best. In addition for Helminth balance I think it would be most interesting to change the Helminth version so that the base numbers on the buffs are 75%(from 150%) base bonus damage and 30%(from 75%) base damage reduction while removing the cap we currently have at 75% reduction and adding a cap of 200% bonus damage. This would make it an interesting but build restrictive defensive tool while not also being an overwhelmingly powerful buff imo.