Mike Simone has developed some custom MiSTer cores that output composite and s-video natively. I go over how to set these cores up.

Links
GitHub downloads: https://github.com/MikeS11/MiSTerFPGA...
Zez Retro Video about these cores:    • How To Hook Up MiSTer FPGA S-Video & ...  
Zez Retro RetroRGB blog post: https://www.retrorgb.com/how-to-hook-...
Instructions: https://mister-devel.github.io/MkDocs...
GitHub: https://github.com/MikeS11/MiSTerFPGA...
Misterfpga.org forum posts:
https://misterfpga.org/viewtopic.php?...
https://misterfpga.org/viewtopic.php?...
Twitter posts:
  / 1509006474508263426  
  / 1509007116274569225  
  / 1509028435783430146  
Antonio Villena’s adapter: https://www.antoniovillena.es/store/p...

Cables and adapters I used (amazon affiliate links):
Keep in mind that Mike Simone is also creating an adapter that will give you better image quality and you wont need these below cables:
Svideo to Composite: https://amzn.to/3OZYgol
RCA couplers: https://amzn.to/390GAZd
S-video to RCA: https://amzn.to/3ylsw6V
VGA to component: https://amzn.to/3waMpeq
S-video coupler: https://amzn.to/37uvH1m

Chapters:
0:00 Intro
0:32 Installing the s-video/composite cores
1:34 Editing the MiSTer.ini file
2:48 Connecting cables
4:38 Enabling S-video on the cores
5:39 Testing games on the core
8:55 Comparing the Antonio Villena adapter