Do you see any hope of a third party emerging, kind of a mix of non nutcase Republicans (a la Cheney) and not nutcase Dems (a la Manchin) ?
Almost none.
The US system was designed assuming there would be no parties. (Remember, they were making up a lot of this as they went along so it is a mix of idealism, practical deals, and guesses about what would work.)
They quickly realized they were wrong, but the "Era of Good Feelings" made it look like it might work out that no parties were needed and so while they fixed a bunch of their mistakes in the first 10-20 years, they never fixed that one.
Anyway, the result is that the system pretty much locks two parties in instead. In one view, that isn't too bad in that basically it just means the parties are the final results of the coalition building you would have in another system - just before the election instead of after.
The whole thing makes third parties incredibly hard to pull off. The last time one existed and actually survived was when the slavery issue split the Whigs up and the Republicans took over. (But notice it went right back to two parties.)
What usually happens is a faction that could make up a third party either tries and fails or just threatens to try and then one of the two existing parties absorbs them. Sometimes that faction takes over.
For instance, Wallace makes a third party run in 68, wins some states, and that opens up the door for the Goldwater wing of the Republicans to make a move to grab those voters. They do and gradually take over the Republicans.
So could you get a "centrist third party"? Maybe, but it wouldn't last.
First of all, what policies would it actually have? "More money for rich people" is unpopular on its own.
The "sensible centrist" is loved by the Mainstream Media but they don't really have a coherent world view other than "these people disagree with people we don't like".
Secondly, even if there is such a group, it would need a regional block to make any indent into power other than throwing votes away in a close race. Is there a natural power base regionally where they could flex some muscle?
Thirdly, one of the other parties would have to split to give them a chance to get anywhere.
One argument has been that the Republican party embraces white nationalism and antidemocratic ideals as their central goal more and more and the other Republicans split off, but what would be the proportions?
It isn't going to be 50/50. What happens to the smaller fraction depends on who they are and where they are located.
I have little hope that your "non-nutcase" group would be big enough to cause a shift. I think there are far more White Nationalists. Sure, if after a split it turned out the Democrats were poised to win everything it could work.
Then the whole right wing of the Democratic party could maybe break off and become the new Conservative party and the White Nationalists drop to the fringe. I just don't see it working out that way.