Videorecording (Webapp, Javascript, jQuery)

A video shall be recorded via html-webapp (mobile, pad, desktop; iOS, macOS, Android, Windows, Linux) and be uploaded to server and converted by server if necessary. The final video shall be mp4-format and receive a thumbnail-jpg.

Ubuntu-Server
PHP8

We are aware of "RecordRTC.js" and "ffmpeg", but facing following problem:
If we upload video-blob as webm from Safari (iOS, macOS), then ffmepeg returns errors converting to mp4. "RecordRTC.js" does not have to be used.

What we need:
- html-webapp (Javascript, jQuery) checking if mp4-recording is possible, otherwise recording in webm:
a.) Requesting user permission to camera/microphone
b.) Countdown (3..2..1) while showing camera-pic (no recording)
c.) After countdown start recording until jQuery/Javascipt-command to stop
d.) Playback of recorded video
e.) 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 - not clientside!)
- creating of thumbnail with ffmepeg (cron-job on server - not clientside!)

Extras:
- adding watermark