Albeit 30 odd years ago now, and undoubtedly some things have changed, I used to live in the Lannion area and cycle a fair bit; MTB and long distant road; even the Bernard Hinault sportive. Long before a lot of these cycle routes, so
@vcballbat advice will probably be better and more up to date than mine.
From memory, I can't remember the Roscoff to Morliax route being particularly enthralling, same with Guingcamp and St Brieuc.
Consider going to Lannion, touring around the Cote de Granit Rose, and then take the train from Lannion to St Brieuc or even Dinan (which is worth seeing in itself). IMHO 4 days with that mileage won't give you sufficient time to see the better places in some detail and at a relaxed pace.
I think I wouldn't bother to actually go and see Mont St Michel on the inside, it does indeed look more impressive from a far. It can be absolutely overcrowded with tourists too like St Malo.