Why didn’t the Priests of the Drowned God ever use their power & influence to gain wealth ? Are they true zealots or is the asceticism a tool of their trade ?

I suppose it depends on whether you care about wealth for its own sake, or whether you’re more interested in power…

As for the Priests of the Drowned God, remember that their entire ideology is based around the idea that ornamentation is unmanly and un-Ironborn unless you pay the Iron Price – and priests aren’t reavers.