A dose of political reality, the base of both parties are going to support the president of their party regardless of anything. Trump is current approval percentage from republicans 93%, from democrats 9%. Biden averaged 88% approval for his 4 years from democrats which is below average of a party’s base approving of the job their sitting president had done. Republicans gave Biden only a 5% approval. Trump’s first term, 92% approval from republicans, 7% from democrats. Obama, 91% from democrats, 7% from republicans. The base from both major parties are going to support the president from their party no matter what. That is just the way it is.
It's independents, the non-affiliated, the less to non-partisan that will move the needle. Not either parties base, their set. Why has Trump this term dropped from a 49.9% overall job approval, 47.7% disapproval back on 1 Feb to today’s 47.6% approval/49.9% disapproval is because of independents. Not neither party’s base in how they view Trump. Independents went from 47% approval/46% disapproval back on 1 Feb to a 40% approval/53% disapproval today.
You’re correct in stating the GOP base will never desert Trump. That is a given in today’s modern political era of polarization, the great divide, the super, mega, ultra-high partisanship. An era we entered in probably during either G.W. Bush’s second term or Obama’s first.