It would be great if there are options that I can play video streams from Netflix, Amazon Prime, Zattoo, YouTube etc. via AirPlay 2 to my 2 HomePod mini in stereo.
You want to stream videos to an audio device? I don’t see any good reason for that 
No I want play videostreams on my MacBook Pro via AirPlay 2 to my stereo HomePod mini setup. Unfortunately with the AirFoil App there is a 2 seconds delay. And the Optimus Videoplayer App doesn’t support videostreams.
Are these files downloaded? Or are you talking about playing them right from the video sites because that’s not going to happen with most video players. Why not just use airplay out built into big sur?
Unfortunately in macOS BigSur AirPlay 2 is only completely integrated (you can use HomePod mini in stereo pairs and without delay) only in these Apple own Apps: Apple Music and Apple TV. Till this day there is only the Optimus Videoplayer App which has AirPlay 2 completly integrated.
In macOS BigSur in the sound setting you can only choose one HomePod mini, but still with 2 seconds delay in all other apps.
You can ask for support in the suggestions section but I doubt it’ll happen for at least a few years. People have been asking for music support for quite a while and from what I’ve seen it’s not even remotely being considered. You’d be better served using something other than airplay 2 as an audio out source.
I think it is a good idea in terms of searching in a collection of video libraries. For example, netflix does not provide “Friends” series but some other providers might do.
You should check out AirFoil for that.
It will stream music to all Airplay, wifi and Bluetooth speakers at the same time.
Not for video streaming. Just audio.