Ouch! That will get in the way of your shootin!
Single action provides an unubstructed sight picture with the long throw hammer so that's good. It also means the trigger press is light - which while less likely to disturb the aim, can allow a weaker grasp. The grasp itself must be firm.
Is this an M&P with magna grips?
My experience with those is a tight grasp with thumb curled down over the top works for me. However I've also added a S&W grip filler (similar to the Tyler grip filler). Depending on your hand, a filler can help a lot with firm grasp. But I think for single action, it shouldn't be neccessary.
With stocks (grips) that are shorter, like the plain Victory stocks, a straight forward thumb may work. If you look at the US Army manuals of that period, that's what they are teaching for the larger M1917 revolver when shooting single action. I have a set of Vicotry stocks but have only experiment a bit with them - and just dry firing. Regardless, what works for me may not work for you.
Massad has a presentation on advantages and disadvantages of different grasps. I'll link to where he discusses the thumb down - fistlike grasp.
Proper Thumb Positions with Massad Ayoob - Master Class Ep. 17 - YouTube
The hurdle is most people today aren't thinking about these older revolver grip types, exception perhaps being cowboy action shooters. That's one reason I look at older material when working with older equipment.
Once thing I do that has been helpful is go to the basement and dryfire (with dummy cartridges). I set a target up against stone wall and use a slight magnifying safety glasses so I can really see the front sight clearly. Often can see faults and correct them and its just a great way to get a feel for the guns.