Query:

I've one core box, all the big box expansions so far, and all the datapacks up to but not including the Lunar Cycle (i.e. all of Genesis and Spin).

It's just me and my roommate playing casual games with no tournament interest.

Is there any actual reason for me to invest in a second core box?

Additional play sets of the staple 3x cards. Being able to run three copies of the 2x cards. Basically it will enable you to build more decks simultaneously and also make your existing decks more consistent. But if you're happy with your current playing field and don't ever play outside your own immediate circle, it's really up to you.