@WhyNot not sure about WiiM as USB audio 2.0 input.
Will be hard to add PCI-E card next to something like RX 4090 monster not too obstruct it's air input, eventually behind it (if such board) or with razer somewhere. In your case you would have to plug it in the last x16 slot.
By proper crossover I mean devided between different chenels with full access to it so you can make it as you wish/need and with high self fillers of course. EQ-APO is free and has a lot of stuff so you should first play with it to get the picture what needs and can be done before baying speakers (with whatever you have already). You will nead a measurements microphone like preferably UMIK-1 and to learn basics of working with REW (free software).
Reconsider easy route (stereo only) with a DAC with inputs you need and basic PEQ capabilities embedded on it at least.
You can go and with stand alone DSP like MiniDSP line's (Flex, Flex HT, studio...).
Regarding sound it's bad, without proper placement of both you and speakers you won't get there. Of course it's very complicated to go other way around for the display and so on. I am in similar sized room and use projector for the purpose on 100" rollable canvas.