@zikohh opera://flags/#ffmpeg-demuxer-everywhere is enabled by default now. There's just no flag anymore to turn it off. So, you're good there.
For opera://flags/#chromium-ua-compatibility, the flag is gone now, so you'll have to use an extension. Install https://addons.opera.com/en/extensions/details/user-agent-switcher-5/ and set it to "Chrome for Windows" for the site. But, edit the string for "Chrome for Windows" to be what chrome://version says for the User Agent in Chrome stable, which is currently:Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.61 Safari/537.36
So, once you take care of the user agent to mask Opera as Chrome, you'll be good to go.