First, with such a rapid fire gun, sit 15+ meters behind cover (bushes or tree tops). Not right beside it, as was done prior to 0.6.4, but 15 meter or further behind the soft cover. You won’t spot a thing, but you can still get radio transmissions from your teammates, and will rely on that.
If you try to snipe the old way, you will lose the camouflage bonus from the bushes/trees every time you fire.
Second, don’t shoot heavy tanks on the front. Don’t bother shooting KVs with that gun on the sides. Concentrate on damaging other, far softer targets, which usually comprise 2/3 of the enemy team.
This will allow you to get to the SU-85B, which at first will be the same as your SU-76 but with more HP and some other improvements. However, when you unlock the final 88mm gun (130mm penetration, 160 damage, high ROF), you will be amazed at how powerful this tier 4 TD is. You’ll still need to employ the same bush/tree techniques (so don’t waste money on binoculars), but you will experience a tremendous jump in combat effectiveness.
And always remind yourself that, as of 0.6.4, and until you’re in a Ferdinand, JagdTiger or Object 704, you’re purely a sniper when playing a TD. Purely in a support role. And that you’ve chosen the hardest type of vehicle in the game, as of 0.6.4.
Indeed, I find that this patch nerfed TD effectiveness so badly that I have not a single TD left in my garage, as I hate the pure support role. Instead I play my Panther, T-44 and T23. And now also the Sherman E8 in preparation for the US TD line, two of which will have turrets and one will have decent front armor, good speed and a good gun, so might be effective even after 0.6.4.