Trying to cut the first few seconds of a MP4 video on a non-keyframe

[Cyber Akuma]

So, this has been driving me nuts. I have a video that I went to cut out the first 30-40 or so seconds without re-encoding the entire thing. Issue is, the part I want to cut on is not on a keyframe, so most software I used either cuts on the keyframe before the part I want (such as Avidemux) leaving in a few moments of garbage video, or gives me a video that is just bank for the first few moments (ffmpeg) until the next keyframe.

I couldn't figure out a way to make anything read the previous seconds of the video and generate a new keyframe at the beginning of the cut, so I figured my next best bet would be to actually split the video on that keyframe, then trim the part I want from the beginning by re-encoding just the half or so segment I want, and then joining that with the rest of the video.

Issue is, I have no idea how to attempt to duplicate the settings of the second half of the video so I can join it. All attempted have just resulted in it rejecting the second half of the video to join (mp4box) or spitting out a video that after that first half second or so is just corrupted and does not play the rest (avidemux).

Does anybody have any suggestions on this? I am assuming the re-encoding route is my only option, but how would I duplicate the settings of the original video so I can re-join it?

[davexnet]

See this thread for suggestions
https://forum.videohelp.com/threads/389413-Is-smart-rendering-on-MP4-files-possible