Finally this ordeal got resolved.
None of the proposed by other users solutions such as Netlimiter 20mbp/s, AV off, VPN region change, deleting fspackXXX files, exiting / relaunching installer etc., etc. worked.
By my deepest surprise (and relief) after more than 20 days trial and errors, the one thing that worked was ASUS ROG Strix Z690 MB BIOS and chipsed/LAN drivers update. As soon as I did that, MSFS installed and updated from scratch at max download speed in just few hours. Go figure…