There are three main factions in the GOP. For lack of better terms, they are the Trumpettes, the Christians and the Establishment.
I have pondered occasionally during this election whether the Establishment Republicans wouldn't get their way fastest by taking their money, their network and their ground game and just walking away from the GOP and performing a coup on the Libertarian Party. Leaving the Single Issue voters behind. The fringe of the Dem party that is available to them would be open to a not-crazy version of Libertarianism that walked the walk of "Fiscally conservative, socially libertarian," I think. Is that enough? Not at first, surely. But over time I think they could pull it off. Spend 4 years building an actually doable platform and convincing existing politicians to switch parties. Run a sane candidate in 2020, maybe lose, and have a shot at 2024.