MPV plugins can actually do frame-perfect cuts and crops for you (+ whatever ffmpeg filters you want), something that would generally require the hassle of opening editing software. And those cuts can be done in h264 lossless (for additional processing later at no additional quality loss from this step).

https://github.com/occivink/mpv-scripts

There is also a way to losslessly cut preserving the original encoding but you give up the precision of the cuts due to keyframes. The MPV script above can do that too: script-opts/encode_slice.conf