Some of your analysis is correct, but merging the left of centre parties to fight the Tories, no.
It's too late for the Assembly elections which are likely to see Tory gains, but there is a good case for a pact between the main parties at the next general election to beat these awful Tories.
I expect the next Assembly to be a supply and demand coalition between Labour and Plaid, with Mark Drakeford, if reelected being a temporary First minister.