I've long favored a third party option, but as you say, now is not the time for that. The only sensible plan forward is repeated, relentless, and hopefully crushing defeat. Only once the Republican Party in its current form is vanquished can we begin to consider changes that may increase the odds of a Republican winning. Ideally "right-liberal" Republicans and R-leaning Independents would vote for the Democrat to have the most effect. At minimum, they need to not case a vote for a Republican.