In the late game, they are bad support the front line, although they have 6 man squad - they die very quickly. Select a commander or spent ammunition to recrew MG/gun is not the rational.
Giving them weapons unlock after teching does not solve this issue. You can give them six LMG42s and they will still die fast to mortar and MG fire, not to mention vehicles that come with late-game progression.
What you'd prefer is durability and other such bonuses like less suppression or even self healing. These make them much more effective simply because they live longer and can keep firing that AT gun.
The only reason you are pressing this is simple: you like to just use these units instead of regular Ostheer infantry. Well, this idea still allows for that without competing with Grenadiers and Panzergrenadiers. More importantly it preserves their intended role as support infantry, which means you don't just use Osttruppen throughout a game. You design a unit like that and it will dominate and replace other units, and encourage blobbing.
So it sounds like you are over-dependent on Osttruppen and seeing that deficiency, you want them to perform better. Not survive better, just be on par with late-game infantry. Problem is, they are not designed that way, and if you DO design them that way they create more problems just so a few players feel better when using them in droves.
I mean consider this: Would the average Ostheer player prefer vet3 Panzergrenadiers with panzerschrecks by the time T3 hits...or an Osttruppen squad that gains the privilege of Russian weapons upgrades by that same time?
Lastly there is the further complication that this unit comes from three different doctrinal abilities; one demands you spend 90 munitions and losing men, another demands 400mp and calls in two squads mid-game, and yet another lets you call them in immediately after choosing commander, and the dispatch rate is very fast. This means getting vet3 Osttruppen with such firepower by the time you go T3 an imbalance...but only if you choose a DLC commander. So to top it off, this also becomes a Pay2Win issue; such a player can gain vet3 Osttruppen and then get weapon upgrades, whereas other ones require munitions and taking losses mid-game to even get them.
TLDR late-game they need buff to survivability not increased damage