Colosseum Forum Visit Strategies

If I'm understanding things right, we can do the Forum area, leave it
and get food somewhere, and the then be at the Colosseum in time for
our 6:00pm entry.

Yes, you can do that. Your tickets should include the Palatine as well as the Forum, and you should visit both of those sites. They are interconnected inside of the security barrier so you cannot leave one and return later to do the other: do both in a single visit. As your entry time for the Colosseum is so late in the day, you might do the Forum + Palatine in the morning, have lunch, and then visit some other interesting places not far from the Colosseum. For instance, these 3 churches are close to each other and not far from the Colosseum:
Basilica San Giovanni in Laterano: one of the 4 papal basilicas, and the "Pope's church" as Bishop of Rome.
https://www.turismoroma.it/en/places/basilica-st-john-lateran

Basilica San Stefano Rotondo: old, rare circular plan + 34 depictions of martyrdom in all of its gory forms.
It's a popular choice for weddings; go figure.
https://www.turismoroma.it/en/places/church-santo-stefano-rotondo-al-celio

Basilica of San Clemente: this one is unfortunately undergoing restoration (it has some fantastic mosaics) but visit the lower level excavations if possible:
https://www.basilicasanclemente.com/eng/
More on the restoration in my recent post:
https://community.ricksteves.com/travel-forum/italy/restoration-work-in-the-sistine-and-at-san-clemente

Just a start?