Brittany in September

I haven't been to western Brittany, but I had no trouble making day-trips from Rennes to Vitre and Fougeres by bus or train.

If you want more than a day long guided tour I did an excellent Road Scholar trip to Brittany in 2017.

https://www.roadscholar.org/find-an-adventure/14250/brittany-and-normandy-distinctive-beauty-proud-ways-of-life

I did post a Trip Report to confess I cheated on Rick, hahaha!

https://community.ricksteves.com/travel-forum/trip-reports/trip-report-paris-brittany-and-normandy-april-26-may-20

I loved this tour and this whole area. I was so taken with Brittany!

I have been in this area, but we had a car. We like very quaint and very small towns, and Locranan (close to Quimper and Douarnenez) was particularly charming. Possibly, my #1 favorite town in France. The coast at Douarnenez was stunning. We went to Quimper and enjoyed walking around for a few hours. I wonder if you based in Quimper if you could find transportation and/or tours leaving from there. It is a rather large town. If a car is a possibility, we really enjoyed the freedom and found it pretty easy to drive in Brittany. We rented from Autocar and it was cheap by U.S. standards and super easy to work with. We were in Brittany at the end of September. The weather in Brittany was on the cool side and it was a bit cloudy (just the way my husband likes it for photos!) We went to Normandy after Brittany and it was sunny and low 70s. I don't know if that is more luck or some kind of difference in climate. I'm sorry I can't provide transportation specifics. Brittany is quite lovely and worth the effort. If you get further north, we just loved Dinard and day tripped there to Saint Malo and Dinan, both really picturesque locations.

It is possible to rely on public transportation to visit many beautiful places in Brittany, but of course it is more difficult to reach some interesting towns, or even impossible for some walks along the coastline.
In Finistère, you could choose a base in Quimper to be able to go to smaller places with buses/trains. Or on the south coast (in Morbihan), Vannes and Auray would be good choices to go to various places rather easily. In Loire-Atlantique, Guérande or Le Croisic are good possibilities too.
On the north coast, Saint Malo is a good place for public transportation with many sites to see within a rather small perimeter.
And of course there is still the possibility to get guided tours, to see differently some (touristic or not touristic) places, you may have a look at http://visite.bretagne.free.fr/index.php/en/excursions.html

We are not maniacs about being in charge of our travel, but we were glad we had a car for Brittany. And we didn't even get south of Dinan, for time reasons. We did do things like small public estate gardens, which were all long drives on 1 1/2 lane paved farm roads, so I don't see any way to to do that with public transportation. If you can possibly swing a car, I recommend it. We rented at a TGV station, not in Paris.

Edit: Perhaps there are Cider harvest events in September?

From Brest, think about getting a driver though your hotel and spend a day visiting the parish class (les enclos paroissiaux). These are wonderful examples of village churches and calvaries in the Breton style.

We spent the last week of August in Kervel, a beach village between Locronan and Douarnenez, and had a wonderful vacation.
It is not the place to go for if you want to lay out on a beach, but if you enjoy seeing the sea, and walking on long sandy beach, it is spot on. And the sunset were fantastic, long and slow.
I second Jules and Monty's comments. I would add the Musée de l'ancienne abbaye de Landévennec and going to the top of Menez-Hom, it has stunning panoramic views of the Finistere area.

Of course without a car, things might be a bit difficult, but not impossible to move around. Here is a link about getting around in Finistere with public transportation.