The Republican party is, clearly, dysfunctional in the extreme and no longer a legitimate party. They don't advance the interests of any but a tiny few and are going all in on white nationalism. But the Democratic Party is far better and totally viable. The main criticism you lay out in the middle there is that they can't ban assault weapons or protect abortion rights, but they have, are still trying, and probably will again in the future... all over the dead body of the MAGA GOP. It's only structural advantages by the Republicans that have allowed them block the Democratic majority as often as they have and I would argue the answer to that is not third parties and more factionalism but stronger support of the Democrats. Look at what Biden did with razor margins in Congress and a corrupt, reactionary Supreme Court. With FDR majorities, we could accomplish all of those items you list that have huge popular support.