I agree it’s a shit situation but the only solution is changing the first past the post voting system. That is slowly happening at the state level but only when Democrats get a significant enough majority to overcome both the centrists and Republicans.

The idea that there is another option besides the Democrat and the Republican candidates is an illusion in all but the smallest races (I.e. water reclamation district boar seats). Splitting the Dem vote is functionally exactly the same as a vote for the Republican candidate.