For your limited use case, ffmpeg / VLC wrapped in a small .sh or .bat script may have been an option?

VLC can understand ONVIF RTSP and tile the video's into a mosaic

https://www.youtube.com/watch?v=qV-S4LMePVk

https://wiki.videolan.org/VLC_HowTo/Make_a_mosaic/

Id love to try lazarus one day all the same