200A service to 100A transfer switch?
Yeah, that's how you do that.

A 200A main panel which contains a 100A feeder breaker to
A xxxA subpanel
via a 100A transfer switch.

The 100A feeder breaker protects the transfer switch.
Assuming the subpanel is quite large (30-space +), it will surely be rated well over 100A.  It will be able to operate safely downline from a 100A breaker.
If the subpanel happens to be what is currently the main panel, well that is just fine! Make sure to rigidly, obsessively separate neutral and ground, and remove the N-G bond.  There should be infinity ohms between N and G, provided the neutral feed line is pulled.
Loads which you do not want to place on generator go inside the main panel.
The only thing I don't like about your plan is putting the 200A main panel on the outside of the house.  Momma taught me not to leave my valuable electrical devices out in the rain.  I would make some effort to seek an inside location.
Also, I think buying a 12-space panel is &quot;throwing good money after bad&quot;.  Nobody ever came onto this forum and said &quot;I am trying to add a circuit and have plenty of spaces in my panel&quot;.  But we hear the other thing a lot.  Therefore I would definitely aim to have at least 45-50 genuine spaces between your main and sub.  And double-stuffs (that fit 2 breakers in one space) are no longer a workable way to solve that, since AFCI and/or GFCI are required on so many circuits, and they don't make AFCI/GFCI in double-stuff.