So i have done some more research and realised im using the term soffit completely wrong. whups. I suppose while *flush mounting* the monitors is ideal. I have two main concerns... I dont have my "final monitors" yet, or even know what they might be, and I may very well be moving in 1-2 years so I want to avoid that sort of rigid construction that cant be recycled/relocated. So I am trying to "half ass it as well as possible" I suppose, putting up lots of broadband polyester treatment that can easily be removed and relocated if I need to. The stuff I purchased is very thick and durable, it will even serve double duty by being a fine material to pad and protect my possessions in a shipping container when I need to move everything, and I can just put them up at the new house without fuss!
There is one rear corner that has a door so it cant have a bulky chunk of broadband treatment in it, I will probably use that area to make a targeted treatment like some panels or limp mass, after I have crammed the rest of the place with polyester and played around with the position of things a bit and figured out how to take a measurement.
I guess I should have put up a visual aid for my room dimensions, I described it in another thread but its probably more useful here too? It is a bit of a strange shape. I should probably mention, I am not an audio professional in any capacity, nobody is paying me anything for the time I spend in this room and nobody will be paying me to use the space ever so I am not expecting or requiring a professional level room, and didnt really expect to get that level of advice from an internet forum to be honest! Its great that people with that level of knowledge donate their time to strangers, and I dont want to seem ungrateful or dismissive of any advice donated to my cause! The truth is that if the room response was critical I would have invested in professional advice from the start.
In my case I just want an audible improvement for a fairly modest budget (~2k), and for the treatments to be of a light construction that can easily be removed and relocated. I am already over budget so I am somewhat limited in how I proceed. There are also numerous, lets call them "domestic concerns" that arise from using one bedroom in a three bedroom house inhabited by myself and my wife, the room needs to be a nice place to live and have space to move around etc. The room gets used for a lot of things, from music production and DJ sessions down one end, to mixing and general computer usage and listening down the other end, and even an impromptu discotheque when the visitors require it! My dogs will randomly do zoomies or have punch ups in here too

So some speaker placements are just not going to work, placing them far away from the wall creates more issues for me that it solves in essence.
If that has you pulling out your hair with frustration I wont tell you that I have another room that is as far as I can tell as close to perfect as you would get in a bedroom for acoustics! If I was really chasing the perfect response it would have been a smarter move to setup in there, but domestic concerns are a thing, that room is nice and warm and quiet so we use it as our bedroom during the winter months and I am not really keen to sacrifice my winter comfort for my audio pursuits.
So basically I appreciate the advice but I dont think I can actually follow through with those suggestions, and I dont expect anyone to provide a professional consultation for free either!
I have decided to treat the whole front wall, theres a layer of 50mm fluffy on it now, and the corners are stacked with triangular offcuts of fluffy with a 600mm hypotenuse. These were basically scrap from cutting the other fluffy to fit the big corner trap. I have two big sheets of acoustic polyester left, I am cutting them so they will take up the whole front wall, and scoring them at the 2/3 mark so they will sort of "fold" into place creating 150mm of treatment for the mid 1600mm of the wall, and the remaining edges will be folded out to create full height corner traps.
I have attached some pics! Thanks
room copy.jpg
IMG_0215 lores.jpg
IMG_0253 lkores.jpg