Create your videos from filmclips, photos, with title clips and effects with kdenlive.
Do video compression with ffmpeg:
ffmpeg -i input.mp4 -c:v libvpx-vp9 -an -s hd720 output.webm && ffmpeg -i input.mp4 -c:v libx264 -an -s hd720 output.mp4
With other options:
ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 48 -b:v 0 -b:a 128k -c:a libopus output.webm
Change the value of -crf to optimize filesize ... bigger numbers are smaller filesize but less quality.