BlueMonkey It sounds as if you are trying lots of combinations of device doing the flashing, OS, browser, and cable, but based on the description I don't feel as if I understand what all of the combinations are or which ones do/don't work.
If you have one Pixel 6 Pro and cable that were able to flash a second Pixel 6 Pro using some browser, I would suggest focusing on that first P6P and that cable and that browser. Thus I would suggest rebooting everything and starting fresh (including checking that the flashing device has lots of free storage, and closing all apps and tabs on the flashing device except for the web installer).