From my understanding, the US has a lot of issues with gerrymandering. There are places where the GOP would have to lose 30-40% of their voters for there to be a change in power. So, within that context, I don’t think what you’re proposing is a good idea. If you tell every single moderate, Trump-hating GOP supporter to leave the party, all that will be left are the fascist alt-right lunatics. People who won’t necessarily lose that entrenched power because of your two-party system. Wouldn’t it be better if more people like OP started participating in the party and taking it back from those fringes?
It seems like you’re also proposing that those people not vote at all. I don’t know if it’s a good idea to promote voter apathy. Shouldn’t it always be the goal to encourage as many people to vote as possible? Especially because your Electoral College also makes it easy for someone to win the presidency without winning popular support.