Regarding plans for Chapter Three, I am concerned about any "rules cyclopedia" being in the DMs Guide.
As a DM, I hope every rule that is necessary to play a complete game of D&D is all in one place: the Players Handbook.
I appreciate how the UA playtests consolidate the skill check rules for social reactions, jumping, and hiding, to be in one place. These default gaming rules belong in the Players Handbook.
The default gaming rules in the Players Handbook must be elegant: "as simple as possible, but not simpler". They also need to be as setting-neutral as possible. A DM needs a free hand to build a world without fighting against any conflicting setting assumptions that are baked-in the Players Handbook.
Any default setting assumptions in the Players Handbook need a light touch. Be "medievalesque" technology but where magic exists. (Perhaps aim for the reallife millennium between years 500 and 1500 CE.) Default classes need narrative flexibility, so the players can use the rules to customize a personal character concept. Likewise, the DM can use these open-ended classes in a new context in a different kind of setting.
All setting details, such as planar cosmology, gods, the pervasiveness of magic items, and so on, belong to the world of the DM. There are official setting guides to choose from that detail a specific world, like Forgotten Realms, Eberron, Ravnica, and so on. Meanwhile the DMs Guide can help the DM create an entirely new setting from scratch.
The mechanical rules to play the game belong in the Players Handbook.
But the details about the setting and the creatures that inhabit it, belong to the world that the DM builds.
Here, all "variant rules" belong in the DMs Guide. The variants can significantly establish the tone and themes of a specific setting.
So if having "rules cyclopedia" in the DMs Guide means, there is a list of tried-and-true variant rules that work well, that is great.
But if the cyclopedia means that I as DM need to keep on juggling back-and-forth between two separate books in order to doublecheck and clarify the gaming rules for a single adjudication, then that becomes unpleasant.
The default gaming rules belong in the Players Handbook, and need to be as simple and as narratively flexible as possible.
Any variant rules that the DM chooses to cherry-pick from the DMs Guide are something different. They are part of the worldbuilding.