Have to re-encode video files, how can I keep maximum quality?

[TopdeckTom]

I'm new to Avidmux and am using it to crop an intro out of .mkv videos. The problem is some are not on I frames so I have to re-encode. What are the best settings to use so I keep maximum quality?

Here is my guess at some settings: https://imgur.com/a/maG7Wp3
Here is some info from the original video file: https://imgur.com/a/Iiz9XCj

[RocketJet]

I personally would tick the "Advanced Configuration" box.
IMHO, it is faster than the "Slow" setting, and compresses better.
Consider trying it out.

I think your choice of 18 for the CRF is good,
but different people have different ideas
for what is a good CRF setting.

[eumagga0x2a]

Your main concern should be to make sure that none of post-processing options in Avidemux are enabled as they will ruin your video no matter how good (how low the CRF) the quality settings of the encoder are.