As far as room design goes, basic understandings of Acoustics are helpful. One such crossover between Acoustics and Architecture would be the Golden Ratio. For brevity I will add links you can peruse at leisure. Short version, Golden Ratio and Optimized Rooms carry some acoustic benefits with the ultimate goal of controlling the room modes as best as possible.
From all the research I have been doing for the past decade, here are a few ratios that I have found that are suitable room dimensions for recording, they are: 1 x 1.6 x 2.56 (famous golden ratio)…
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The Cardas Speaker Placement Guide. Setting up speakers in a golden ratio, golden cuboid, golden trapagon room, etc.. Speaker placement calculators.
www.cardas.com
Of course this only really helps if you are building from scratch.
Acoustic treatments be they structural or added in after is a whole other can of worms.
Depending on the ultimate space, I would still advise upgrading your fronts to Silver 500s and the C350 (don't recall which center you already have). You are on the right track for Subs... but again, keep in mind that the overall room volume is what is most important for the Subs in matching to the room. The PB3000s would be minimum I would consider.
Cheers!