@theiconer1 since my inital post I made some interesting observations. As it stands, the root cause is not the Nextcloud server side, but weak performing client software!
On my PC (Windows 10) I usually use Firefox, and as it appears, Firefox is performing week when it comes to high bitrate video streaming. I could solve the issue simply by using Chrome instead.
On Android, the official Nextcloud App can’t well stream high bitrate videos either. When I use the NC App just to find out the URL to the video - but manually paste the URL into Android VLC Player, the video is streamed seamlessly. Oddly, when the Nextcloud Android app is used to stream with VLC as external player (tab on the 3 dots next to the video filename, then select “stream with”) the performance is as weak as when using the app’s build in player. It looks when involved, the Nextcloud Android app causes a general bottleneck that drives (high bitrate) video streaming unusable.