A unit that is available at the early stage of the game and then adapt to all stages of the game becomes spam-able and leave little use for other units to bring something to the table.
Like.... panzergrenadiers?
Or... Panzerfusiliers?
The solution does lay in making Penal able to do everything but to have different units do different things so the army composition is diverse.
Guards PTRS for penals it is then.
Why in you opinion adding a separate anti vehicle infatry in T1 is a worse solution?
Because it fixes problem that isn't there, creates more unit clutter, impacts manpower vs muni balance, requires additional work on vet and whatever would be invented here, you still wouldn't be happy, unless it was some shitsquad to the letter by how you imagine it.