I don't know how exactly to do this but I think a good buff to Penals would be to do 2 things.
1. Make the AT satchel deal less damage but have more range.
That's called AT nade and they already are available for stock squad + AT satchel is not offensive weapon, its a deterrent to pushing them.
2. Lock the normal satchel and AT satchel behind the Grenade Package in the HQ.
Why?
You are adding an extra cost here, what is the point of that? Regular satchel can not be used against infantry anyway if your opponent isn't browsing xvids while playing anyway.
The increased range doesn't need to be much but it being a little easier to get a snare off would be nice to see. Plus the biggest change is that penals can have a snare without needing to get PTRS rifles which means there is a better threat of snares being something the enemy has to watch out for. Currently with only 1 penal getting PTRS (you can only afford one realistically) the amount of snare Penal starts have is laughable. If any penal could snare it would make the job of the PTRS a lot easier overall.
Penals are not supposed to have snare without PTRS.
The whole point of AT satchel is to keep vehicles away from them as they lack alpha damage to scare away med armor from engaging up close.
Also, if you need this much snares, don't spam penals, build cons.
Another idea is to do a bigger set of changes to the SU76. Currently there is just little reason to get one compared to the T70 and after that point they just don't preform well in any way.
What if you made the SU76 like 60 fuel and toned it down to mainly make it effective against LVs and even handle (with vet ofc) mediums? It could essentially become the mid game of a penal start and be the core AT of that build that helps against infantry thanks to it's barrage ability.
So maybe the change could be something along the lines of:
Fuel cost reduced to 60 fuel (from 75)
Penetration values decreased (practically a Pz4 would bounce a good amount of shots from an unvetted SU76)
Vet changes (things like Vet 2 granting more Pen)
Any changes to Barrage to balance out the cheaper cost (I would assume this takes a smaller nerf to help prevent spam being to strong)
Then you'd kill the unit completely as it would pretty much revert it to sate from 4 years ago, because there is exclusively ONE unit it would be able to engage - puma.
I have no clue if the above changes would be enough nor would I know the exact values they should be made to but it could be another way to improve penals by giving them serious AT to shut down LVs in a more serious way and also has the added benefit of making the SU76 more appealing to get in a build.
Ideas about giving them lend lease AT are not new ones, but I don't think they'll ever get it.
SU-76 will never be appealing over T-70, because SU-76 does not plug the hole T-70 does - complete lack of AI in early mid game.