All good points. I have read that a perfectly square room is about the worst acoustic situation you could be in because of equidistant walls and standing waves. Acoustic treatment could improve things. I would consult some experts (maybe GIK), because I wouldn’t know how to treat this type of room.
In your equipment, I think the weak link are the cables, especially the speaker wire. I have used the Blue Jean Beldon speaker cables and they are pretty much low fidelity. Even the basic 12awg stranded oxygen free copper Monster Cable / Monoprice stuff is a lot better. You can improve upon that with some solid core Audioquest type or other similar cables.
The interconnects can be improved as well. Tons of opinions, but I would look first at some of the Audioquest interconnects due to their solid-core design.
I would say that the Blue Jean Cables Beldon 1695A coax (18awg solid-core) would make an excellent interconnect for the subwoofer.
Since streaming is 75% of your content, maybe look into a USB reclocker, such as Wyred 4 Sound Recovery. Or a USB-to-SPDIF convertor:
https://www.computeraudiophile.com/forums/topic/15026-15-usbspdif-converters-shootout/
A Schiit Yggy could be an upgrade, but it’s expensive and will take up a lot of your budget. I think the cables would really be a first step and would return more benefit for the money you spend. Then, if you had enough left over, look at a used Yggy? or something similar like LKS MH-DA004 or an Audio-GD device?