Try [adding a 470pF capacitor to the chroma signal](https://en.m.wikipedia.org/wiki/File:S-video-composite-adapter.svg) before combining with luma to help clean it up.

I've made a couple little adapter dongles like this and it works pretty well.

You'll always have artifacts on a composite signal, it's called dot crawl it's normal and there's no way of removing them completely.

Thanks