Unable to find anything remotely close what I am trying to achieve of registering a custom entity extending a custom render

It should be in entitytypes

Yes, but tracking is necessary.

I propose a two-part approach, not unlike what we do with ToolManager:

• Add a TrackableEntityTypeRegistry, which has a Map<EntityType, EntityTrackingEntry>, or something of the sort.
• For most people, add a FabricEntityTypeBuilder which acts like EntityType.Builder but adds a .trackable(...) option, which automatically registers in said registry.

https://paste.asie.pl/SaMR

Here's how to make a chicken.