Long term audiophile seeks room acoustic experience...,

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

It’s the best money you can spend for sound improvement, IMO.
ASC or GIK (manufacturers of acoustic treatment) will send you a proposed treatment plan for a small fee, refundable on your first order. It’s not a bad way to get started if you have no experience reading about room acoustics (and I don't mean reading forum posts, including mine).

It is possible to overtreat a room with too much plain absorption. To avoid that, apply the treatment in stages and evaluate frequently. IMO it’s critical to avoid using much plain absorption, in favor of products that combine absorption and scattering (e.g., GIK Alpha panels, RPB BAD panels, any panel or trap made by ASC).

Avoid advice from anyone who says, "Product XX from manufacturer YY blows everything else away" or "Product ZZ can treat your room with just a few small dots / cones / pucks / gongs / mini panels." No manufacturer has reinvented physics.

Good luck!

Whats most critical. In your mind?

Achieving critical mass. It is really hard to hear 2 or 4 panels alone having a big difference in a room unless it’s truly very small.  Consider at a minimum 6 units.  Two corner bass traps and at least 4 more panels.

I would say the front of the listener, in a hemi-sphere, is what’s most important (unless your seating position is right up against a wall).

The floor and ceiling between listener and speaker are among the reflection points to consider.  The floor between and behind the speakers.  The corners.

I've just gone through this -- teaching myself for the last 7 months. The advice on this forum is excellent. I've learned a lot.

In addition to the advice here and in books suggested, I really got reliable information from Anthony Grimani's youtube interviews on Audioholics, some of Acoustic Fields more basic explanations, and the website Acoustic Insider. Also, GIK's websites are helpful.

A few additional things which have worked for me:

1. Get REW software and a mic. Learn how to use it. It is like a flashlight in a dark attic. I used it first to check on listener position and speaker position. This is a complicated process but if done systematically on REW -- interspersed with listening -- it can help start you off on a sturdy foundation.

2. Don't buy cheap treatments. 

3. Deal with 20-300 hz first. It's the hardest.

4. OC 703 is hard to find locally at Home Depot, Lowes, etc. Be ready to buy online. Even then, it's not that expensive. I covered mine in fabric and skipped the wood frames. Rock wool (roxul) is also good. Local insulation companies (where contractors go) may have it in stock.

I'm a newbie at this but would be happy to chat with you. Message me if you'd like to talk.