@technomicks Download the Opera GX 64-bit offline installer from https://www.opera.com/download. Then, close Opera.

Then, uninstall Opera (don't choose to delete your data when uninstalling).

Then, launch the Opera installer, click "options", set "install path" to "C:\Program Files\Opera GX", set "install for" to "all users", uncheck "import data from default browser", check "Make Opera the default browser", adjust any other options, and install.

Then, Opera will show up in Default Programs in Windows and you'll be able to set it as the default browser.