I don't see any meaningful way to talk about that hypothetical. Much of what Trump says is chosen specifically to enrage people, and Democrats in particular.

Even if he chose to support, say, universal health care, it would be buried in so much deliberate harassment that there would be no way to arrive at a workable plan.

The suggestion is so counterfactual that I just can't address it one way or the other. He achieved very little legislatively because he didn't really want anything except to anger people. The one thing he achieved is by appointing people to a lifetime office that isn't subjected to checks and balances -- and that accomplishes everything he might want.

Before Trump won the Republican nomination for 2016, during one of the debates against Rubio and Cruz and whoever else had not yet dropped out Trump actually brought up universal healthcare as a reason to support him over Cruz and Rubio, saying we'd get it under him and not under them.

No they wouldn't. There'd be a lot of Republicans who would be running around telling everyone how universal healthcare is actually a good thing now, but any Democratic concerns would mostly be tied to quibbles with process and taxation, not the healthcare itself.

I'm pretty sure if Trump supported universal healthcare, people would get on board.

The problem is 1) very few of his actual policies were good on the face of them and 2) even some of the milquetoast stuff he did had unsavory motivation.

Realistically, any theoretical universal healthcare proposed by Trump would probably have strings attached that further eroded the functioning of the federal government (which is a common theme between pretty much all R legislation).