I’m not sure where to start.
I shoot 120 grV because it shoots moa for me and because it is the max charge recommended by Western (Hodgdon) (I have smokeless muzzleloaders but I shoot smokeless in those) - NOT because a 209 will not ignite more than that. You wrote, “You need the heat of a large rifle primer to burn more then [sic] about 80gr of BH209 (by weight).” This is ABSOLUTELY NOT TRUE!
LRPs and LRMPs don’t produce a larger flame volume than a 209. They may be cleaner but they aren’t more powerful and they aren’t needed. Hodgdon recommends the use of CCI 209M or Federal 209A primers. The theory, in part, behind 209 alternatives is that they disturb the bullet/charge less and thus are more conducive to accuracy. You can also argue that depending on the action and plug design, they may handle more pressure (as in some smokeless) or be cleaner (less leakage or flame channel buildup). As I said, guys that shoot competitively (and win) at 1000 yards (like shooter and custom rifle builder Jeff Fisk) nearly all use 209s with their BH209. To quote Jeff, “I personally believe the federal 209a is the absolutely [sic] best primer for Blackhorn. It’s all I use.” Lots of guys, me included, have tested bigger charges of BH209 and have NO ISSUES with ignition or barrel fouling with 209s. There is really zero reason to use anything other than a 209 or 209a with Blackhorn! Doesn’t matter how big your charge is.
And while we’re at it, 84 gr by weight of BH209 may or may not be 120 gr by volume. Depends on which lot of powder you’re talking about. Recent lots of BH209 have been getting denser so 120 grV may weigh as much as 91 grW from the current lot. But you probably know that….