i don't think it's as simple as 'covering bases' to be going up against a well-resourced party structure on its own terms and expecting change. to me that is an unrealistic expectation.
There are weaknesses in most structures, you just have to find them. The right-wing takeover of the USA has demonstrated this, even if it did take them decades to accomplish it.
i'd rather see challengers running under a different party, i just have zero confidence in the democrats at this point.
There have been other parties for quite some time. They're lucky to have even 1% of the population's awareness, however. America's electoral system virtually guarantee it'll stay this way, which is why I see disrupting the Democratic party from within as the more likely to be viable approach for the time being. We can work towards progressives splitting off into their own party at some point, but they need to get bigger first or they'll lose and take their reputations down with that loss.