What about if breaching was less about instant damage but more about forcing an enemy out of a building?
Hear me out here; the breaching ability basically becomes this:
You pay muni to "breach" the door and enter into close range with the enemy. There is no special damage incurred to the enemy besides what would normally be dealt to the enemy at point blank range. The enemy is not forced to be inside the building with you except for (maybe) a second or two so that the breach wasn't a complete waste of munitions. Then the enemy can decide to leave the building or stay and fight it out.
This way, it's not an instawin button against garrison troops.
I am actually not a big fan of the mechanics which includes any CQC fighing inside buildings (while realistic, I dont belive it will be good for CoH gameplay), but I agree with you regarding breaching being more of a force enemy out of the building rather then being damaging tool.
I would rather see breaching being cheaper and over-all weaker then the regular nade throw, but if breached enemy squad should be forced out of the building and maybe have some sort of "stunned" debuff, while breaching itself should deal more of a static damage being like 25-40% of garrisoned squad HP.
In this case it would allow it to maintain original idea behind it (being a way to deal with early key point garrisons) while also there would be a clear distinction: want to deal more damage? Throw nades. Want to force out enemy out of the building? Use breach.
But still what really concerns me - pottentual no brainer abuse. Since, alright garrisoned MGs are cancer in CoH2 on some maps, but when non MG unit uses garrison its somewhat alright. So what if you just managed to garrison lets say 2 squads in different buildings and enemy is just blobing on you with 4 squads.
Nothing really stops him to pretty much deny you your garrisons and better prepared possition with just 2 breaches, almost instanly. I mean sure, you might lose this engagement even without breaches, but they are allow your enemy not only save him MP (since he would have lost more models fighting off garrisoned troops) but more importantly - time. Engagements which might have take 30-40 seconds, now only takes lets say 10-15.
At this point it would have been easier for you to just stay in green cover and avoid buildings completly, since with breaching its actually easier for the enemy to deal with garrisoned troops, then with the ones who are just stay in green cover.
Garrisons are already kinda a grey area zone. Game impact they have is directly tied up with map desing, number of windows, directions of mentioned windows, size of the building and so on. Breaching brings the idea of buildings being something so strong that game needs additional mechanic to counter them, while in reality they are 100% situational and map dependant.
Realistically if mappers didnt slap huge full-HP church in a key areas or buildings near enemy cut-offs, then there would have been no need to moan about early game garrison rushing. I mean sure, when the we place super strong building, in a super strong possition then no wounder it will be hard and frustrating to deal with.