Writer Carlos Cisco has indicated that they conceptualized the Breen’s solid form as an adaptation that they developed over time
Interesting.
It’s completely unrelated to the current Disco story arc, but that does make me wonder if the Vorta and other Dominion species would have been aware of the Breen’s goo form. The Changelings evolved the ability to shapeshift over time, but were originally solids. having the Breen start out as the soundtrack producers for the “Spider-Man & Venom: Maximum Carnage” video game, and later develop a solid form might seem anathema to the Founders and their worshipers.