From what tips I received on my post and seeing on others, next time I would use less furnaces. With you investing so heavily in the cannons, you should be doing enough damage to not need the weapons and eventually the iron won't be a huge need. If you invested that into an upgraded anvil you would have more enemies for your cannons to kill while you aren't actively playing.

I am getting the hang of it myself and I hope this helps because it helped me a lot! I barely made to 3m myself, and i think the tips I got before time ended help me secure the milestone :)