My hors is like this 100% and tbh we’ve never resolved it completely, however getting someone to work on his TMJ worked wonders for this, for the most part, regular Chiro and body work specifically geared towards the TMJ and head.
I found bits like Nathe and HS Duo seemed to be the best with my grinder, I had luck with the Acavallo Sensitive Bit, and things of that sort.
I’ve had my horse 13-14 years. I bought him as a 4/5 year old greenie WB. He’s now 19. He still, on occasion, grinds when he’s bored, displeases, etc. It’s a habit more than anything at this point.
ETA: background