You can try modifying Opera's desktop shortcut or pinned taskbar shortcut to pass the --no-default-browser-check command-line option to launcher.exe to see if it helps. If that helps, that'll at least prevent it when you open Opera yourself via the shortcut.

Besides that, you could regularly (you'd have to experiment without how long you can go) edit the "Preferences" file in Opera's profile directory (while Opera is closed) with JSONedit and make sure the root/browser/default_browser_infobar_last_declined timestamp is recent by updating it to the current value at https://www.epochconverter.com/webkit. That might keep Opera from bugging you. There's also a root/ui/launches_until_default_browser_check value you can try setting to a high number that you'd never reach to see if Opera honors it.

Another option is to just using a standalone installation of Opera. It can't set itself as the default browser and won't ask you. Launch the installer, click "options", set "install path" to a folder somewhere in your user space, set "install for" to "standalone installation", adjust the other options as you want, and install. Then, create a shortcut to that Opera's launcher.exe and or pin it. You can look at its profile/data folder to see the structure. You can then delete everything in the data folder and copy everything over from your regular Opera 's profile and cache folders to get all your settings. Then, you can uninstall your regular Opera.