I could see them getting a decent return if they retained even like 25%
You have to retain the entire length of the contract. And you're only able to retain on 2 3 contracts at any given time. So that would mean the Flames would be without 1 of their retention slots for the next 5 years.
You can retain on 3
Yep, you're right according to CapFriendly. I've corrected my comment. Thanks for the correction!