How do I scale Plage Bearer?

Besides prism of belief (+3), crown of victory (+1) support gems (+2), gemling (+1) and reg passive tree (+1).

Is there other ways to add levels to Plague Bearer? The scaling is insane on it
It's a buff not a spell so +# to spells doesn't work on it sadly.

I'm really enjoying building around it. Using it + corpsewade is a joy.

No one really?

GGG dropped the ball on "buff" type skills. No reason why my +7 chaos skills staff doesn't work on plague bearer. :| only due the fact it is classified as a "buff" and not a "spell"

As you say, the scaling is insane. If they did allow it to scale with spells, they'd have to adjust the scaling. So I'm not sure that dropped the ball is the right way to frame it. I am not aware of any ways beyond those you listed, based on the lack of responses that may be generally true.