That drawing was surprisingly empty. You've scrubbed about everything you can without altering your database. I proved this out by hunting down the MapsDbArchive dictionary entry in the DWG itself and manually deleted it with a VLA-DELETE function. When I saved the drawing it was 193kb.....

So, I would definitely say it is time to do some house cleaning on your database. Maybe export some legacy stuff out to IEZ's in case you need it again later, but ultimately delete things your not using from your database. I am also a big fan of making as many things multipurpose as possible to prevent excessive duplication. Another thing that may help is a complete separation of your piping and duct databases; although your piping database by itself will still be very bulky.

You are correct though, it isn't the number of ITM's, but rather all the various other things: Materials, Specifications that reference those materials, Services, Service Templates, Connectors, etcetera.

I tested this in both 2016 & 2017 with the latest service packs and found the same. Note that when I didn't open it with original database and saved the file I got the same result, but when it said no to the original database prompt, it then cleared the crud out. So, you may consider just doing an IEZ on everything in your projects services and importing that into an empty database, then working on that from that point on.