That path doesn’t involve the DNC. Look at how they treated David Hogg, of all people.
Unless we have a left Tea Party. Just look at how successful the Tea Party was at forcibly taking over the RNC.
The tea party had a lot of astroturfing. Again we look at how Sanders, AOC, and Hogg are treated to see how the structural power of the part will be turned against those that push back even a bit on their insular geriatric views. I see value in what you’re describing, I think I just see transforming the democratic party as more of a trap of energy than causing the needed change that people actually want.
A third party with our FPTP voting system will cause an inevitable spoiler effect, which will lead to Trump winning his inevitable third term run (if he even bothers with an electoral process and doesn’t just instill himself as king).