Take a look at this site Formule golf which lists just about every golf course in Southern Brittany - and if you book through Formule Golf there are discounts to be had.

I wouldn't claim to be an expert but you seem to have covered all the courses you could walk to. If you want to extend your options to about a 10 Euro taxi fare (each way) then:-

Odet - about 2 miles or so from Benodet
Ploumer Ocean - about 10 miles or so from Lorient
La Britesch (sp?) quite close to La Roche Bernarde which involves a nice trip up river after locking through the Arzal Dam
Rhuys-Kerver - a couple of miles from Port la Crouesty.

I played Cournouaille (La Foret), Odet and Rhuys-Kerver last week. The first two are in really good nick and well worth playing, Rhuys-Kerver was very wet for a links course and a bit featureless - very long holes a lot of which go straight up and down.

Ploumer is always a good test of golf but beware slow players making it a very long round and La Britesch (not part of Formule Golf) is very attractive with a few of the holes circling an impressive Chateau.

Hope that helps