Ubiq: I don’t believe there’s the ability for a write-in candidate in presidential elections, since in actuality you’re not voting for President. You’re voting for electors. The 538 electors are the only ones who actually vote for President.
Anyways, for the nomination I would certainly vote for a Republican who is better than Trump instead of Trump. In fact, I did exactly that in 2016 when I voted for Ted Cruz.
But at the end of the day I think it is more important to nominate an acceptable Republican who will win the general election rather than a more ideal candidate who will lose the general election.