I believe your choice of 135* would be fine. I normally just leave the seat as the drill cuts it. Being all the primers I have seen have a non square corner, a rounded corner to seal against the seat, gas cutting is going to happen as long as there is leakage. If a rifle has a bolt to close against the primer it can be set to slightly crush the primer for a better seal, but in the case of the break open guns, there isn't anything to cam the primer in tight. Being primers of the same make very in length, the o-ring might be the bestest possible solution. The o-ring doesn't seam to cause a problem by not allowing the action to fully close, it is very forgiving. And that should help stop the cutting and help with the fit of varying length primers.

I have always drilled straight threw and then tapped the hole to 10x32 for the vent liners. This is how the Savage plugs are made from the factory and they hold up great. they are made of I believe 4140 chrome moly steel the same as CF rifle barrels. I have not found any flame cutting in the savage plugs, but the primers are crush fit in these guns. My Remington Rolling Block that I built for smokeless use has breech plug made from 5/8 Grade 8 bolt. This rifle has fired over 800 primers and there is still no sign of gas gutting in the primer pocket. Because this rifle has a rolling block it closes very much like a break action gun, it only holds the primer in place no crush fit.

I recently complete a conversion on a Remington 700ml to sealed 209 ignition. I fit the bolt with a 25 ACP shell holder to hold the primer. I bushed the firing pin hole in the shell holder to .120. The I turned the last .450 of the firing pin down to .110. this is similar size to a shotgun firing pin. I modifed the Rem breech plug by fitting it with grade 8, 3/8 x 24 bolt. this alowed me to make the primer seat and fit the plug with a Savage style vent liner. ( I drill my own Halo Chrome 10x32 x 3/8 screws to make vent liners) I fit the plug to my batch/lot of Winchester primers so that the shortest primer is crushed .002 and the longest would be close to a .010 crush. Before I returned the rifle I shot it over 80 times and had no blow back and allot of fun shooting. The owner of the rifle stopped by to pick it up and we shot it 25 more times that day, he was thrilled with it's performance and I showed him how to drill out the breech plug to clean it. The rifle arrived to me with a #11 nipple in the breech plug. it left with a blow back free 209 ignition system that will fire BH209 every time. This rifle had been neglected and even thou it was SS the barrel was badly pitted, the BP was even badly pitted. I couldn't believe how good this neglected rifle could still shot using 90 gr V BH209 and Harvester Sabots with my cast 333 gr LBTs or even with TC Cheap shots and the same load.