I would remove the PTRS rifles and adjust the guard mosin to compensate – making it similar to conscripts at close range, but with much less drop off over distance.
This would solidify guards as a long range defensive squad, with a clear vulnerability to vehicles and CQC squads. With no AT damage, only buttoning, they cannot function alone against vehicle play.
So I'm pretty confused by this proposal. You want to make them a long range defensive squad by removing their PTRS (which perform well at long range and require them to be stationary to fire), giving them more of a rifle with insane moving performance, and nerfing said rifles performance? That just makes them more mobile, and a lot weaker at range.
Like, I'm all for making them less of a generalist anti-everything squad, but your proposed solution doesn't really match your proposed goal.
Change the accuracy bonus from to the last man to more RA and ability cooldown. Allowing them to take more risky pushes to get close and do demolition duties is more important than their raw damage output.
To be fair, the glass cannon niche isn't really common in this game. Falls and penals are probably the only infantry units that fit that design. At the very least, having them be glass cannons keeps them unique. Beyond that, RA doesn't really make sense to me for a squad meant to take losses, that would generally just mean that they become durable and end up not taking losses. It can make you put them into situations with more damage being thrown around, but it probably won't make them riskier to use or make them better thematically. High received accuracy means the squad WILL sustain losses, and to the last man means you at least have some incentive to use them in a way that makes them sustain losses. Again, I'm all for making them more of a high loss squad, but I don't think the proposed solution really pushes that direction. Side note, 4 ptrs would probably too much for reasons that other people have brought up.
Reduce the veterancy accuracy bonuses on Cons. The default con rifle being so bad but getting huge vet spikes causes big issues with captured weapons. Instead, accept that a weapon upgrade taking up a slot is just the more sensible way to let conscripts scale.
Cons accuracy bonuses aren't anything special (its penals that have insane acc bonuses). They get 40% which is relatively common. Its their received accuracy bonuses that are irregularly high. There also isn't an issue with captured weapons because (again) cons accuracy bonuses aren't anything special, they have an accuracy penalty on slot weapons, and they only have one weapon slot (iirc). If you wanted to say that cons need weapon upgrades because them relying on vet bonuses necessarily causes issues, you haven't really established that.