----launchbrowser=0

I assume you meant --launchbrowser=0.

1. I wrote a script with multiples winget update in a row, beginning with some specific overrride options, then doing the global update.
2. Trying with the last update the mentioned parameters in the --help option with the installer. apparently you need to add -- in front of the options to be functional (it wasn't clear)
3. Winget accepts --override only with one package at a time

So, it's my final version in my script :

winget update --silent --accept-package-agreements --accept-source-agreements --disable-interactivity --override "--setdefaultbrowser=0 --desktopshortcut=0 --startmenushortcut=0 --pintotaskbar=0 ----launchbrowser=0 --silent" Opera.Opera
winget update --silent --accept-package-agreements --accept-source-agreements --all --wait --disable-interactivity 

If this doesn't work for you, my last solution is to block the version used in Winget so it'll ignore any future update.

The installer parameter copyonly=[0|1], is usable too but not sure of what append after this kind of install.

See the "--singleprofile=1|0" section at https://reddit.com/r/operabrowser/wiki/opera/installer_commands/ for one example of copyonly.

Haven't messed with winget and don't really know how it's all set up, but there might be a way to pass --setdefaultbrowser=0 to the winget upgrade command you use to upgrade Opera. Or, if you do winget upgrade --all, there might be some file that you can put --setdefaultbrowser=0 in for Opera so its upgrade command passes that to Opera's installer.
But, that's assuming winget makes use of the Opera installer at all. If the Opera winget package is just Opera's install files that winget just extracts or something, winget would need to support not making Opera the default browser on upgrade.
Something to investigate though.

For Winget you have to use the --override parameter.

For example : > winget update --override "setdefaultbrowser=0 desktopshortcut=0 startmenushortcut=0 pintotaskbar=0 launchbrowser=0" Opera.Opera

The installer parameter copyonly=[0|1], is usable too but not sure of what append after this kind of install.
: If true, the installer will only copy the files to the installation folder and will otherwise not perform any modification on the system.

For the > winget update --all I use > winget update --silent --accept-package-agreements --accept-source-agreements --all --wait --disable-interactivity --override "setdefaultbrowser=0 desktopshortcut=0 startmenushortcut=0 pintotaskbar=0 launchbrowser=0" and as many installer ignore unknown parameters it works most of the time.

I tried uninstalling Opera and reinstalled it with 'Make Opera your default browser' deselected, but the moment it got installed, it was the default browser again. I didn't delete user data at the time of uninstalling but the way Opera is behaving these days, not sure even that is going to help it.

It looks like this is done not by the installation itself, but by Opera when you run it the first time after it updates. When I ran Opera after the latest update, I got multiple Windows prompts telling me that Opera wants to make changes to my device. I allowed the changes and then checked 'Default Apps' in Windows and confirmed that Opera became the default browser again.

Haven't messed with winget and don't really know how it's all set up, but there might be a way to pass --setdefaultbrowser=0 to the winget upgrade command you use to upgrade Opera. Or, if you do winget upgrade --all, there might be some file that you can put --setdefaultbrowser=0 in for Opera so its upgrade command passes that to Opera's installer.

But, that's assuming winget makes use of the Opera installer at all. If the Opera winget package is just Opera's install files that winget just extracts or something, winget would need to support not making Opera the default browser on upgrade.

Something to investigate though.

I would download the Opera installer, goto the URL opera://about, take note of the "install" path and close Opera. Then, uninstall Opera (do not choose to delete your data when uninstalling). Then, delete the install folder (it will be left behind after the uninstallation). Then, delete everything in "C:\Windows\temp" and "C:\Users\yourusername\AppData\Local\temp".

Then, launch the Opera installer, click "options", uncheck the option to make Opera the default browser, review everything else, and install.

Hopefully that will fix things where the updater won't try to pull a fast one.

Opera just kept updating itself as usual, and one day it started setting itself as default browser after each update. Every time I have to manually revert the default browser using 'Default Apps' settings in Windows 11. This started happening around the same time as the splash screen and the startup sound were introduced.