I think the way to go with cons would be remove some utility then give a weapon upgrade.
I mean, a mosin of extra dps is a mosin of extra dps. A model of extra durability is a model of extra durability.
Anyway, I'll give my reasons why extra models and cost reduction is better than a weappn upgrade.
Cons are obviously unique among mainlines in that they do not have access to weapon upgrades without commanders. A small part is at least keeping that small unique aspect. Also, back during the games release, 6 man squads were genuinely unique and an actual identifiable strength of each soviet unit. With the release of osttruppen, paras, and panzerfusiliers since then, this part of soviets/cons has kind of lost its exclusivity. Other factions can get similarly durable squads, and so cons lost that selling point. A 7th man at least brings that advantage back to the table.
2. Cons as a utility unit
The one thing that most people seem to agree that cons have as a strength is their utility. A weapon upgrade would probably shift this identity to that of a more generic mainline. Yes, they would still have utility, but they wouldnt be a squad thats about their utility anymore. More so, giving them a weapon upgrade would end up disincentiviszing the use of merge. Basically, the opportunity cost of using merge goes up since each con model is worth more combat power. An extra model and reinforce on the other hand incentivizes the use of merge. You save even more manpower when merging, and can merge more since you have more models to spare.
Edit: Forgot to mention that the extra model makes their construction time for sandbags faster. Also, for the few doctrines that have con repairs, itll also make them repair faster, as minor as all this is.
3. A reason to use them instead of penals
If we make cons a part of the dps race, then the choice between penals and cons becomes less meaningful. Unit with dps and some utility #1 (cons), or unit with a little more dps #2 (penals). Cons are something of a combined arms option becuase of their ability to merge and build cover. At grenades are the kind of thing you pair with a zis to fend off vehicles, while ptrs and at satchels are the "do it yourself" option. The 7th man upgrade and reduced reinforce caters more towards cons combined arms identity than a weapon upgrade would. And, as opposed to penals, who are supposed to operate and win by literally winning engagements, the con upgrade helps cons win not necessarily by making them win a given engagement, but by trading more efficiently that your opponent.