Starting off three days in Nantes and then going along the south coast, Crozon then inland Huelgoat way.
Not bothering with north coast this time.
There are some superb wilding locations around the Golfe du Morbihan.
You may consider Quiberon and the standing stones around Carnac worth a visit. The aire in Carnac is very popular, but I was able to get in at about midday in mid-August; others arrived during the afternoon but in peak season an evening arrival might require a fair bit of luck; not sure about the situation there in April.
The river Etel is reputed to be particularly beautiful, but not sure what wilding spots may be found there. There is a chargeable aire reported reported to have views of the (sand) bar.
There are numerous small coastal aires (no services) in the commune of Névez (SW of Pont-Aven); services available at the aire in the bourg itself. The coastal aires range from Port Manec'h in the east to Raguénez in the west.
The ville close in Concarneau may also be worth a visit. Some decades since I last visited.
I'm less familiar with the west coast, but there ar a few good locations around Plonévez-Porzay.
If you go anywhere near Mûr-de-Bretagne, the Lac de Guerlédan has a number of very pleasant overnighting locations.
In addition to the WC POIs,
CAMPINGCAR-INFOS is an extremely useful resource.
If you don't yet have a Crit'Air sticker, it may be worth applying before your trip. I can see no indication as yet that it is required in Brittany, but you will be close to departments 49 and 85 which are covered by the scheme and additional areas are being added. I would imagine that it is only a matter of time before Nantes or the entirety of department 44 is added.
Hope the above is of use.
Tom