The 2016 and 2020 elections both had historically high turnouts.
2016's turnout was 55% of eligible voters. That's not historically high. Clinton underperformed Obama in total votes received.
2020's turnout was historically high- it's tough to say whether that was all anti-Trump energy (in which a ham sandwich with a (D) next to its name could have won, or if it was all pro-Biden energy that no other Democrat could have received (but TBH, I kinda suspect it's more the former than the latter)
Would Bernie have been able to get more votes than Biden, then follow it up by passing as much impactful legislation (e.g., the IRA) as Biden did? We can’t really know
Probably not, given that centrists seem to prefer kneecapping progressives to supporting them.
As for things we "can't really know", we do know 100% that Clinton didn't win in 2016, and that resulted in flipping SCOTUS rightward for a generation, the overturn of Roe, it meant that we'd have the pandemic under leadership that just wanted people to pretend it wasn't there and sacrifice themselves for the economy, it was a terrible shit-show and the biggest thing we all got was ballooning debt so the billionaires could get their tax cuts and American foreign policy experienced setbacks from which it may never recover.
He’d be labeled a full commie
So was Biden. So was Obama. So was FDR. So was Kennedy. So was LBJ. They've called every Democrat to the left of Hoover a communist since Woodrow Wilson's administration. This "oh no, we have to nominate people that republicans will accept or they'll call us names" nonsense is quite possibly the worst sort of preemptive-surrender politics imaginable and I imagine it has something to do with why young people don't vote