yes I think that is a bad idea on opening up the flash hole there, it allows gases to go both ways and coming back at you can be dangerous
when I had a few custom plugs made(by my gun smiths trying to find a way to get gun to shoot)
we did a slightly larger flash hole on the bore side, just a little to get more a flare on things as it left the plug into the powder charge
DIDN"T help me at all, or sure didn;t notice any difference in anything
but again, out o ALL my plugs and I had a bunch, I never had issues with delayed charges
I however was also pulling breech plug about every dozen shots or so, and cleaning flash hole
so, things always went off instantly as they should!
I do recall some primers were WAY dirtier than others
its been so long I honestly don;'t recall what one's were the cleanest and what I settled on
as between me, Thompson center and two custom rifle builder that always gave it a go trying, we went thru about every combination that existed
SO if it was me, I'd start with a NEW breech plug here to be SAFE
and from there try some NEW primers, make sure there fresh and dry fire off some primers of a few brands and see what one's leave your flash hole the cleanest(also check chambers for how dirty just a primer charge is!(no charge in barrel here, just firing off primers and inspecting things!
and then maybe stick with the one that seems to keep flash hole the cleanest!
and then once that is figured out, start shooting things again(making sure charged are seated well and see if any better
Also, any chance the hang fires are primer/firing pin related?
did you save any to see if lighter impacts/dents on primers?