You'd think if FFMPEG has a copy function that doesn't re-encode then it shouldn't touch the frame rate either, but it does, even if I specify the frame rate. However Resolve evidently only likes constant frame rates in the imported clips (long story short it ****s up the videos). Ultimately the reason why I need to convert these videos to MP4 is because I need to edit them in Resolve which doesn't support Matroska. I ended up trying FFMPEG's copy function too, which would have been good because it doesn't transcode, but due to the way these encoders all seem to be unable to handle constant 23.976 frame rates 100% accurately, it ends up producing a video with a variable frame rate. The error it chucks out is also rather vague so I've no real clue as to what's wrong there. Initially I found the tool MkvToMp4 which out of all of them seems to be the one I got on well enough with and it worked for a time, but it hasn't been updated in 7 years so is obviously no longer supported, and a handful of videos that I need to convert do have EAC3 audio tracks, and it fails whenever it tries to convert them (it just creates an MP4 video without any audio).

I'd rather just use the individual tools myself. Do I firstly need to use something like mkvextract from mkvtoolnix, then for the videos with EAC3 audio tracks use eac3to to convert them to AC3, then use MP4box to package them as an MP4? Or am I missing any vital steps and tools there? I know there are plenty of GUI tools that do all this in one frontend without the need to use tools individually but I've found the ones I used were either outdated and barely worked or were just too confusing to use because they do too many things.

How to make a mkv file into mp4