Finally I fixed it
It was almost freezing when I worked and the brake oil seems much less fluid at low temp, that's probably what explained the issue (worsened when I slightly push the brake paddle).
I completely removed the full calisper, which gave more manoeuver room to use an appropriate tool and gently push the pistons successfully.
Lessons learneds:
- try to always keep the other brake pad when you replace one
- more difficult to work when it is really cold