Here are a few tried and true steps you can take to figure this out.
Sit in your listening spot. Clap your hands. Listen closely to the echo. Does it decay nice and smooth and uniform? Or do you hear a flutter? If the sound trails off nice and smooth, how long does it take? Do you hear the sound in the room die out, but the echo coming from the 7x7 opening lingers longer? All these little things you are hearing, they are telling you where to focus your efforts.
Now clap and listen from different places in the room. If you have parallel walls go stand near one and clap. If the walls are parallel and untreated you will probably notice some flutter echo. Instead of trailing off smooth the echo flutters. This is the clap sound literally going back and forth across the room, a fraction of a second each time, so the echo comes back and forth so fast it flutters.
Now in a small room, any room really but small ones are worse, if you try and treat this with all absorption the room can become dead. Also it will be real easy to kill the room you are in making the sound coming in from the opening all the more noticeable. This is a balance and another reason no one here can tell you what to do.
I know all this because like you said I made all the mistakes and learned from them. Had my room all treated and sounding great until I realized it was too much. Took a lot of panels down and it was better. Still have a lot more I can do. Diffusers will help a lot. Even with a dedicated room it takes time and money, I have only so much, and you can see where and what I have prioritized. Stuff like Synergistic Research HFT and Schumann generators are way more cost effective than most room treatments, and are practically invisible. You could get a FEQ plop it down in a corner and have more improvement than hours and money spent on GIK or whatever.
If you do want to try panels go to the hardware store, get some Owens Corning 703, just one 2x3' panel, cut it up and try clapping and listening to music with it in different places. This costs next to nothing. Not as cheap as advice here, which is free. Either way you get out of it what you put into it. https://systems.audiogon.com/systems/8367
Sit in your listening spot. Clap your hands. Listen closely to the echo. Does it decay nice and smooth and uniform? Or do you hear a flutter? If the sound trails off nice and smooth, how long does it take? Do you hear the sound in the room die out, but the echo coming from the 7x7 opening lingers longer? All these little things you are hearing, they are telling you where to focus your efforts.
Now clap and listen from different places in the room. If you have parallel walls go stand near one and clap. If the walls are parallel and untreated you will probably notice some flutter echo. Instead of trailing off smooth the echo flutters. This is the clap sound literally going back and forth across the room, a fraction of a second each time, so the echo comes back and forth so fast it flutters.
Now in a small room, any room really but small ones are worse, if you try and treat this with all absorption the room can become dead. Also it will be real easy to kill the room you are in making the sound coming in from the opening all the more noticeable. This is a balance and another reason no one here can tell you what to do.
I know all this because like you said I made all the mistakes and learned from them. Had my room all treated and sounding great until I realized it was too much. Took a lot of panels down and it was better. Still have a lot more I can do. Diffusers will help a lot. Even with a dedicated room it takes time and money, I have only so much, and you can see where and what I have prioritized. Stuff like Synergistic Research HFT and Schumann generators are way more cost effective than most room treatments, and are practically invisible. You could get a FEQ plop it down in a corner and have more improvement than hours and money spent on GIK or whatever.
If you do want to try panels go to the hardware store, get some Owens Corning 703, just one 2x3' panel, cut it up and try clapping and listening to music with it in different places. This costs next to nothing. Not as cheap as advice here, which is free. Either way you get out of it what you put into it. https://systems.audiogon.com/systems/8367