Videorecording (Webapp, Javascript, jQuery)

A video shall be recorded via html-webapp (mobile, ipad, desktop) and uploaded to server.

The final video shall be mp4-format.
Ubuntu-Server, PHP8.

We are aware of "RecordRTC.js" and "ffmpeg", but facing following problem:
- if we upload video-blob as webm (iPhone), then ffmepeg returns errors converting to mp4.

What we need:
- html-webapp (Javascript, jQuery) checking if mp4-recording is possible, otherwise recording in webm
- playback of recorded video
- upload video after click on "ok"-button (creation of filename shall be done at receiver-php on server)
- if webm ... ffmepeg-command for converting to mp4 (cron-job on server)
- creating of thumbnail with ffmepeg (cron-job on server)

Extras:
- adding watermark