I could explore another way to add custom armor: https://github.com/Ancientkingg/fancyPants
This should work for everyone without Optifine. I still need to try if I can combine it to support both...

Someone just told me that MC 1.20 would also support a custom armor system. I might want to use that instead.

The custom armor system of mc 1.20 seems to only support a limited set of variants and colors, so I will try Fancypants first.

This has been implemented in KCI 12