These are all good points, and I really appreciate and applaud a well-reasoned answer. The polls were not as far off in 2016 as you remember. In the week before the election, Trump closed the gap pretty quickly. The RealClearPolitics average had Hillary up by 2% in the national popular vote, and she won by 2.1%. That’s an average of all the major polls, which is a lot more reliable than any one poll.
The point I was making about approval ratings is that Trump’s numbers are down significantly in every state, relative to his numbers when he took office. It doesn’t matter for the argument whether the numbers are accurate now. It just matters that they’re much lower than they were when he was inaugurated. That means that his support has slid by quite a bit. Whether people who disapprove of him will still vote for him remains an open question. But his support is lower than it was when he was elected, which suggests strongly that he’ll get fewer votes, all other things being equal.
If anything, the polls in 2020 will underestimate the Democratic share of the vote, because young voter turnout is likely to be a lot higher than in 2016, and those voters aren’t represented to the same extent in the polls. Some of them weren’t registered in 2016, so they don’t get the calls.