Is it possible to build a category system? I have read that the tag system can be used for that in some post but not more.
Yes, tags are ReactionCommerce’s equivalent of categories. You can assign tags right away to your products in admin UI.
Has RC Innately the capabilities to group variants so I could group colors and shoe size or would I have to add that capability via a plugin?
No, there’s no need for a plugin. What you’re looking for is called ‘options’ and you can define and edit them in admin UI as well.
What is the current state of the integration of Dynamic module loading and is there an issue which I can follow?
Can’t say much to that one, except that it seems to me that not all resources are lazy-loaded right now…