Find me a single flaw in this logic.
“If you keep doing the same thing for 30 years and end up with a more liberal party than when you began, who is now fully supportive of leftist policies on immigration/borders/homosexuality/marriage ect, you should stop doing that”
Just one.
I'm with you 100% in spirit, but until someone builds a party machine to equal the existing Republican mechanism voting for anyone other than the Republican in a general election will accomplish absolutely nothing positive, and will only lead to more Democrats being elected. It's not complicated - that's the only possible outcome, unfortunately.
That's why the only rational strategy is to use the primary system to change the party from within. It's also important to get involved in the internal party machinations, beginning at the caucus level. If enough conservatives got involved in the party itself, instead of sitting in the corner and pouting by casting meaningless protest votes, things would change.