>>1553796
ffmpeg

VP8 or VP9
https://github.com/Kagami/boram
Or just post mp4

>>1553796
https://github.com/dfaker/WebmGenerator
https://github.com/argorar/WebMConverter
Those 2 are great, just input the 4 or 6 MBs depending on the board and click convert.

I use shotcut

>>1553796
I use mpv (Linux and Windows) with the webm.lua - allows you to make webms (or other formats) on the fly while watching a video.
I also use Avidemux or Shotcut if more editing is needed, such as merging or dubbing.
>what type/settings does a 4chan WEBM need
for file sizes per board, see https://github.com/4chan/4chan-API/blob/master/pages/Boards.md which links https://a.4cdn.org/boards.json

>>1553807
>>1553810
Since you use shotcut, you may like this course:
https://kemono.cr/gumroad/user/6792186152658/post/tixrdc

You may already know how to use it well but it will teach you to do some cool things.

>>1553796
Doesn't the site support mp4 now?