Like L'Oiseau we sold our french house last year, with an old nonconforming fosse. But which had worked fine for us all the time we were there.
"I suspect my (French) buyers, sensibly, will do nothing about replacing it until they are absolutely compelled to."
But to the OP, yours sounds a bit more complicated. Especially the location of the fosse.
You need to weigh up the costs. If you like the place so much, that means a lot.
I guess a new fosse septique costs about 10,000 euro to install.