That’s a great question and one that’s always difficult to properly address because it’s difficult to know which needs the most immediate attention. On the one hand, if you’re committed to building a great system and taking your time, I would start at the foundation and get the Power Plant and work my way up.
The reason I put it this way is that the biggest immediate bang for the buck is likely going to come from upgrading your phono stage as the Stellar phono is startlingly better than the Schiit. And room treatment is even more fundamental but without knowing if you paid attention to room setup (as opposed to treatment) it’s near impossible to guess from a forum post what to do.
I would also advocate grabbing a copy of my setup book if you get a chance.
So, foundation first, then phono preamp for a real thrill.
With room treatment I’d start with the bass and then do the rest. Adding absorption will drastically soak up frequencies above 150hz so do it a little at a time unless you’ve got EQ capability.
Also, related to room treatment and understanding the problems with your room, I highly recommend getting a USB microphone and downloading REW (Room EQ Wizard) or equivalent analysis software. This gives you a very clear picture of aspects like frequency response and RT60 (reverberation time) that will inform your treatment approach.
As mentioned above ideally you’ll also have some means to alter your systems response via EQ.
Build some DIY bass traps. Not all that diffuclt, or expensive. Then treat side wall reflections with fabric wrapped OWENS CORNING 703 rigid fiberglass 4’x2’ panels.