There seems to be a winning strategy that is way too easy to perform. If you simply surround ALL the buildings with streets, they never end up combining and creating the harder to manage multi-store apartments/factories.
You have way too many streets available and this is too easy to perform. I would either change it so buildings can grow no matter what (IE they don't need a partner building to combine to grow) or make it so that always selecting the street gives diminishing returns.
In one example I ended up with 6164 points. I didn't use a single bus, tram or subway. I simply created streets around every building and the game basically played itself. I doubt this is intentional.
EDIT: Another thing you could do is just start spawning the bigger buildings directly. Do not spawn them by merging two of them exclusively. Try to first make them like this and, if no combination is available, spawn a big one directly to punish the player for doing this.
You would need to maybe adjust the spawn algorithm to make sure that in the spawn before the merger you try to create a nearby, otherwise bad spawns would punish players without any reason.