Hi,
I live in France & my Baycruiser 23 is based in Lorient. I have lots of experience sailing between Brest and Pornic.
The best place to launch is opposite Lorient (Locmiquelic) roughly 1h45 - 2h from Roscoff. You are very close to the island - Groix, Belle Ile and the Golfe du Morbihan or you can also turn north to the Glenans Islands and Concarneau. Lots of small rivers to visit (Odet, Aven, Belon ...).
The marina at Locmiquelic has good launching facilities with a pontoon next to the launching slip. The staff at the marina are very friendly and helpful.
The concret slip is quite large, there is a good parking space to prepare the boat and it is not busy !
I also know a boat yard in Lorient where you can store your trailer.
The other launching sites south of Lorient (around the Golfe du Morbihan and La Trinité) are very busy during the summer period and parking is a real problem. Southern Finistère is another good option (Port La Foret) and you are a day's sail from the Morbihan
Don't hesitate to get in touch for more details & sailing ideas.
Stephan