The offer was just last year. You probably can go back to the table with them to talk about the offer they made and if they're still interested.

Or, shop around, which is time-consuming. If there are multiple offers, it's up to your group to decide.

Or, this may not the option you're looking for but, i think, this is your BEST option....

SHIP IT!

Only then you'll know how much it's actually worth. Look for ways to manage it - outsource the management of the product if it needs some backend support. Maybe find another co-founder who has the time to run the product.