Thanks for the reply.
Actually I first installed Vaultwarden, and then when I realized it was a 3rd party app, I start to do some digging to see if there was a 1st party app.
I contacted Bitwarden support (as a premium user) and they explain that the steps described in their documentation should be similar on my NAS.
I also found some blog posts (here and there) that explain the procedure.
I’m having some chat with the support, but I also decided to try my lucky over here, maybe someone knows what I’m getting wrong with my conf…