Another update!
I worked the horse in a bit for about 20 rides. Tried different styles, tongue pressure, bar pressure, bar pressure with tongue relief, plastic, metal, Myler. I would let him wear it free longing for a session, then a ground driving/long lining session, then riding. Ultimate, I found he was always trying to flip his tongue over the bit, gaping his mouth (no noseband) and the feel in my hands was always resistance/anxiety at the start of a downward transition. I tried a shanked hackamore with padded noseband and had very little control. Went back to my modified rope halter and found that had the best feel and response. All this to say that I think the horse does have some discomfort with a bit.
Now for the good news! After much searching, I found a vet near me that does a manual float. I sent over all records that had references to sedation amount and horse’s reaction. Vet was suspiciously confident, but he came out and hand floated with minimal sedation and minimal drama. Vet is kind of an unusual/unpopular vet in the area. He was very secretive about the drugs he used - said they’re his own “special concoction”, I got the feeling he’s protecting his trade secrets, but the procedure went smoothly so no complaints from me. This vet recommends semi-annual hand float (fine with me) and thinks he might be able to get the wolf tooth next spring. Recommends staying bit free until wolf tooth is removed.
In the meantime, I also have a Myler hackamore on loan from a friend that I will try, it’s a shanked hackamore, noseband looks like chain covered in leather sleeve. Sounds severe, but I need something on an athletic 6yo WB if I’m going to ride in the field this winter.