Sounds similar to my situation...
- existing 100A SE panel in house
- existing 60A sub-panel for basement suite
- existing 40A sub-panel in attached garage
- going to install new 200A service in attached garage
- going to install a sub-panel in detached garage
I've found that standard 100A and 200A panels (with main breaker) are very cheap because of the market demand. OTOH, a 200A enclosed switch, splitter/gutter, and a 200A load center (no main breaker) are going to be spendy.
I'll be installing the 200A SE panel in garage, then feed the house, suite, attached garage, and detached garage from it.
The only situation I see where the main switch is desirable is if all your loads are closer to the existing 125A panel than they are to the new SE. Even then, I think I'd scrap the gutter idea, and feed the 125A as a branch circuit in the new 200A panel. Heck, if you keep with the same brand panels, there's a chance you can convert your 125A panel to main lugs and re-use the 125A breaker in the 200A panel.
I've intentionally not gone into Code details as the NEC applies to you, and the CEC applies to me.