The only thing here that was not obvious six months ago is that Trump would get the Republican nomination. Sanders could never have successfully governed. He would have made Carter look like a great success — which corollary to you own points. The idea that the other Republican candidates might have been tolerable is just wishful thinking.
The long game? The only long game is take over at the local level and move up the roots of the tree. Any other idea you have about taking over the Democratic party is just another fantasy. So the question is: are you really willing to work for change? It is not a short term project, it is a twenty-year or longer project. It is however more important than Pokemon Go.