I hear you, brother. FWIW, I think there were many things that appealed to people and what got him elected in the first place and continued support in 2020.
1) He was not part of the swamp as you allude to. After the arrogance of the Obama administration and of Hillary Clinton, many people felt the elite ruling class (including a segment of the Republican party, people like Mitt Romney) thought they had it figured out and the country was their oyster. They could do what they pleased and while they were not able to complete a leftist shift towards moving the country down the socialist road, they were well on their way; Hillary's election would seal the deal.
2) The news media is dominated by elitists who look down their noses at the middle class and, particularly, flyover county. I'd personally like to tar and feather Jim Acosta. Trump stuck his finger the eye of both these groups and people loved it.
3) He was goal oriented and is results driven, damn the niceties. Unfortunately, this was his biggest problem, he never seemed to really understand how to play Washington politics to really keep his policy goals on track. Just because he and the Senate Majority Leader of his own party may disagree on something is not a good excuse to attack said Senate Majority Leader.
4) Most people I know hated his Twitter account. Only a few I know felt it was the only way he could talk directly to the people. Personally, he made me despise the words Twitter and tweet. But it was sometimes fun to see him stick it to the elites.
5) The massive support in 2020 was push back against what, with very few exceptions, everyone thought was a senile moran being led around by a bunch of woke idiots and some puppet masters we really do not see (his cabinet is absolutely full of clowns and I don't believe they're collectively capable of running a lemonade stand with any competency, let alone the Federal government).
6) His actual policies appealed to people who supported him (I did for the general) like illegal immigration, recognizing China's economic hegemony, ending the endless wars in Afghanistan and Iraq, Israel, recognizing and calling out the European NATO countries riding us for their defensive umbrella, the understanding what a freaking high cost government regulations are and lowering income taxes, especially corporate income taxes and his calling out the fact that Washington now thinks it owns the country, doesn't serve it. There are probably more, these are just ones off the top of my head.
I hope he decides not to run again as I agree with you, he is seriously flawed individual for that position. If he does, I'll support someone else in the primary. If he is the nominee, I will support him in the general because the Democrats are being led by full-blown idiot leftist progressives.