Well, the bang for buck is definitely DIY. GIK is very popular. I have both GIK and Real Traps. I prefer the latter's product, but its MUCH more expensive. Of the typical products between the two its usually heavier and/or more massive with Real Traps, with better fit and finish IMO. You also can choose between HF versions, or regular. I got them as used, but even then its still very expensive.
Your room looks very modest, so Id look towards bass trapping as a pretty big priority. Maybe some GIK tri traps in the front corners. Maybe even all corners (because IIRC you love your subs and have three of them?!). Then for reflection points, the sidewall is the first Id be looking at due to the very close proximity of the speakers there.
From there, Id experiment with panel(s) at the front or rear. Even perhaps the ceiling at some point.
But bass trapping is of large concern right now I think. Remember, you don't have to do it all at once. Add a couple here and there over time. Eventually, the cumulative effect should help out the room quite a bit. Make sure to check the classifieds regularly. Acoustical treatments are kind of hard to abuse, don't "fail", aren't "refurbished", or out of warranty. So they get eaten up real fast on the classifieds. Gotta be quick when you see them.
