Small room-Front diffusion vs. absorption

Welcome to the forum.

It's good you have some absorption and plan to include more. I will say that diffusion does not make sense behind your speakers, at least as a first choice location. My reasoning is two-fold:

1. Diffusion is mainly for frequencies between 4-8kHz. That bandwidth is both highly directional and likely to be either absorbed or masked before making its way all around your room, into the diffusion, then back to you to create any meaningful impact.

2. Diffusers, because of reason 1, tend to be relatively shallow. The only way to make diffusion meaningful behind your speakers is to make the wells very deep so they can function on lower, less directional, frequencies. Even then, you're better off treating that range with broadband absorption anyway.

I hear you asking about absorption and diffusion and how it relates to a room's size. This is complex but I take you to mean, "Do I need to be worried about diffusion in my small room?" My answer is, "Not unless it's part of a comprehensive strategy."

Lastly, you're asking a bunch of questions about diffusion. These signal to me that you're not precisely sure about what diffusion is. If this is the case, then I suggest forgoing diffusion for now and focus first on positioning your speakers and listening position ideally, then adding sufficient broadband absorption and then positioning that ideally. Only then should diffusion come into play. If, and when, you decide to include diffusion, you should expect precision and expertise in its construction.