Hello,
it is easy to get lost with all the x264-options. Fortunately, x264 comes with presets. To use them in recent avidemux 2.6.3.x, I did an effort to simulate at least three of them. See attachment.
You have to place the files to the location where the other *.json files are situated. This should be in a subfolder ("x264/1") where you find other user stuff of avidemux:
Linux: ~/.avidemux6
Windows: %APPDATA%/avidemux (could be avidemux6)
After choosing one of them in the application, you may adjust the crf-factor according to your needs. I personally use crf 14 at the moment (with preset veryfast).
oops, missed that one