I believe these two APIs are very similar in what they do and could benefit from a common API (if possible).

Don't you mean IClientItemExtensions::getCustomRenderer as an equivalent to Fabric's DynamicItemRenderer?

I'm not very familiar with either API right now, if getCustomRenderer is closer to DynamicItemRender then that should be used, there's also a deprecated Fabric BuiltinItemRenderer which might help support old Fabric versions that don't have DynamicItemRenderer.