Room treatments behind or between speakers?

It's room dependent, and Milpai's treatments are a good example of absorbing bass then getting better imaging by using a middle panel.

Most acoustic products are made with fiberglass. If you're on a budget, you can buy fiberglass panels (not the big rolls), and use them for room treatments. If they work out, you can cover them with fabric so they look nice

A lot of people put diffusers on the wall in between speakers.

Use mirror method along the wall to locate position of the treatments.

Since your room is small, you will most likely get a reflection off the back wall which then bounces off the front wall and then back to you. I would think absorption on the front wall between your speakers and maybe try some diffusion on the back wall behind you. You could try that and also reversing the combination to see which works best.

I have no treatment on my front wall since my 31 foot long room is a long enough distance that the rear to front wall and back to me reflection isn't an issue. I've been told that at that distance the time delay of that reflected sound is great enough to become a non issue.