I think you are confusing Trumpists with Republicans. Remember that a significant part of Republican party went with Trump reluctantly... they still don't really want to be the party of Trump.
There are true believers who will defend Trump no matter what. The key will be when Republicans start bucking.
I think there's very little chance of formerly anti Trump Republicans migrating to any of the announced Democrat candidates, or to anyone who might emerge ( during Biden's ongoing twilight). This, I believe, is largely the result of the "oppose Trump at all costs on all issues" behavior of the current Democrat Congressional leadership, coupled with the evolving Democrat Party platform for 2020, led, as it is, by a very left wing DNC and the emerging new left in the Party itself.
Basically two significant things have changed since the last Republican Primary in 2015: firstly, Trump's policies have proved to markedly improved our overall economic performance: the stock market has boomed; work force participation and employment are much improved; wages are rising and consumer confidence is up. Secondly, the Democrat Party has moved very far to the left since the 2016 election. It is no longer the party of 2016, and increasingly the new elements of the Democrat platform are driving away even long term Democrat loyalists. The reactions of most labor Unions to Medicare for all is a telling example.
During the early 2016 Primary I was much put off by Trump's egoism, vulgarity and then novel use of public media and other direct public communications. Indeed I started out as a Romney supporter, but was finally frustrated with his feckless indecision and, at the same time, amazed by the effectiveness of Trump's new approaches. What has followed since then has amplified both my initial concerns, and my ongoing desire for effectiveness in implementing policy. Meanwhile the Democrat alternative has become far more unappealing to me. I believe this characterizes the experience of most of the "moderate Republicans" to whom you referred.