Solo queue, i usually play support or dps when playing with friends. You’d be surprise what some light comms do at even low ranks. Bronze to silver I mostly played Orisa/Soldier so long ago it’s all a blur, then silver to gold was where the Zarya grind started. I didnt really play much Zarya while i was in bronze, it was primarily me playing Orisa and plopping down on the objective and hoping for the best. Even though I didnt play much Zarya in Bronze, I assume it would work fine, because i could carry in pow silver just fine with her. As for people not shooting bubbles, i just bubble either my mtank who is probably feeding or my flanking dps who is about to start feeding, or a frontlining Brig. The thing i learned about carrying as tank in low ranks, is that if you are just your average player trying to climb, you cant really just go Hog and Ball and carry. For your average person, it is best to enable your team to do stupid stuff by playing aggressive and yelling in comms to hold w. People need to give low sr dps at least some credit, most of them are high level players who know to ult a graved enemy team and are just low rank because they dont counterpick. And with general focus fire not really being a thing, fights last really long in low ranks, so building charge is the least of your worries. I guess dva would also be a good carry tank at low ranks because bomb is basically a guaranteed teamwipe in bronze, but you are stuck playing dva, who is pretty useless without a coordinated dive, and is a very tough hero to play, not to mention that after Bronze, no one really dies to dva ult anymore. Bastion is less of a problem than you would think, because even if the whole team plays around the bastion(which is rare), low ranked players dont know how to keep the hold, especially once one of your dps farms an ult, or if they are really that bad, you farm grav.
This is it for my memoirs of Zarya in low ranks.
Tldr: Zarya is good because she enables your team to be stupid with bubble and doesnt require a ton of skill to use effectively. Also, remember both teams are bad, so if you give your dps the opportunity to pop off, they will. Use comms to yell at the team to hold w harder and combo gravs.