I did it: I followed the steps from the link, option 2(https://www.reddit.com/r/operabrowser/wiki/opera/linux_libffmpeg_config/), but instead of creating a new folder as mentioned at the end of the tutorial, what I did was replace the one that Opera already had with the one I took from Vivaldi. I hope it helps someone else.
Unfortunately, you can't install opera stable, beta and developer at the same time from the repos.
I had to install opera stable 98 from flatpak, beta 98 and developer 100 from the repos, but now I can't upgrade to beta 99 (manually, not in the repos yet) because of conflicting files (see https://forums.opera.com/topic/62356/opera-99-beta/3?_=1684003600733 for details).