> and any life present at the destination will not use the same DNA coding for protein (if gene expression even works that way).

Well, that could be worked around in the world building. My favorite SF-friendly scenario would be if life originated in the Sun's natal cluster (perhaps not around the Sun itself), with tens of thousands of star systems, and spread between them before the cluster dispersed. Presumably panspermia would be much easier in such a situation because the stars are closer together and because maybe residual gas could help particles get trapped near other young systems. In this case all the "infected" systems could have the same coding.

A nice consequence of this scenario is it's compatible with the Fermi argument: even if origin of life is unlikely, it just had to happen once here, and so it not happening elsewhere in the galaxy (or even visible universe) is not a problem.