Feel free to post any insights you have as well. I'll be editing this initial post every so often
2-24-2022: Got up to a really high population--almost 500,000--in global warming age, but traffic became a BIG problem once I replaced the last of my low-density housing with HD res, so much so that my population decreased to around 300k. It seems that having a bunch of hospitals and schools isn't enough--if they're at the corners of your city, traffic will get really bad. This is the first time I've almost exclusively had hospitals and colleges at the corners and it's also the first time I've had so much traffic problems. I'd assumed that having them anywhere would simply raise the health and education levels, but it seems the sims have to actually be able to go there in order for that to happen.
2-25-2022: Did a quick test and it took 54 seconds for the game to pass 6 months in the Global Warming Age. For anyone who wants to try Future% (even with cheats) you're gonna have to do a LOT of waiting, even after you're done with the main story.

Traffic can be fixed with bus stops in the intersection so roads don't cross to remove traffic. Then the lack of traffic won't cause pollution spots making land value faster to grow bluer and more sim would live there. Also less roads so the game has less places to put traffic and more building space.
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for me, this is not an issue. Taxes are the real issue. Try reducing the tax(as long as you get income) from 8-8-8 to 7-3-3 in 220k, 2-2-2 in 350k, 1-1-1 in 450k, and 0-1-1 in 550k population.