I would go with slick tires first, and lights if riding at night. A backpack for carrying stuff.

After that I would ride for a while before deciding what else I wanted to change. You have experience commuting, so you'll know what you like and what you don't.

I have found that spending money to upgrade stuff that didn't need to be upgraded...
Well, I have a box of stuff that will probably never be used again or didn't get used to begin with.