After so many trials, it looks the win11 reinstall seemingly solved the issue.
I have consistent measurement with Edge and speedtest desktop app.
Pleasant surprise, all my MSFS settings were saved under my live account, so no need to set it again after a full reinstall.
It is bad I had no idea what caused the issue, but it is good working now and can fly the PMDG in less than an hour the fenix A320 as well 