Unit balance isn't likely to change much anymore.
Elephant already got a big price hike. (It used to cost 400ish you know)
And if you're just slugging it out face to face Elephant against ISU-122, you're doing it wrong. The counter to the elephant is infantry (and airpower if u have superiority), even in open terrain. You can get 10 concripts for the price of 1 elephant, they move just about as fast, and there's no way the elephant can handle even 1 conscript in a 1 vs 1 engagement.
Besides, the ISU-122 has the bonus of having an artillery mode.
Opening up SAS map by removing rivers and making more crossing will fix this issue. Elephants and their high price and slow speed should be much more easily out maneuvered.
SMG infantry isn't supposed to be an 'all purpose make all other Soviet infantry obsolete' unit. They serve a very distinct role of being close terrain fighters, but it's unavoidable they are somewhat overshadowed by the new and improved 43 variants of the other infantry. SMG infantry are a 1942 infantry unit after all.
Maybe a price reduction is in order, but that's about it.
I'm not overly convinced the air war is that bad. Seems to be only buying Yak 3's is the problem.
You can get 2 I-16s for the price of 1 Yak 3. So instead of 2 Yak 3, get 1 Yak 3 and two I16s. Use 1 I16 to soak up interceptor fighter, use the remaining I16 and Yak3 to annihilated the frail German Stukas.
After you hit and run the Stuka, retreat any of your surviving aircraft, fix them up, and repeat the process.
This will leave the German player with 2 options: Buy enough fighters they can destroy all your soviet planes in a single turn (at least 6, probably 7 fighters to 100% destroy these 3 soviet units). Or continue to lose Stukas every time the Soviet player does their hit and run.
Or a Soviet player can lull their enemy into a false sense of air superiority, hiding their fighters for many turns, eventually the opponent will get sloppy and stop escorting their bombers and stop concentrating all their fighters, and then that's when the Soviet player returns in force. I've seen this happen many times on Hylan Valley. The player who initially loses air superiority regains it about 10 turns later.
Also, the extreme durability of the IL2 makes it a pretty strong asset, it almost doesn't even require a fighter escort, leaving the Soviet aircraft to go full bore into the Stukas.
As for making more units more useful, this was already done pretty much to the limit. Any further and we're just going to piss off the 'historical accuracy' folks.

Making more units more useful doesn't mean making all purpose 'uber' units (which some people seem to think is the answer, but will only lead us back to the original problem)