hi hi! just chiming in with some extra miscellaneous things i did/learned while setting all of this up for myself and a friend~
first of all, maybe i just missed this because i'm sorta dumb?? but there's a REALLY important step to do in the sdvx asphyxia plugin, which is setting the file path for the game and then running the "update webui assets" thing! the game works without all of this, but using the asphyxia web part is miserable and also a few hundred songs will likely be missing (mine was like this, at least)
second, for the whole audio situation, i had pretty good luck using the (accursed) voicemeeter application, basically i just set a virtual output to be 16bit 44100kz, and then in the windows sound settings, i make sdvx use that output, so i don't need to change my actual settings. then in spice i set the audio mode to direct (instead of asio), and turned off all of the audio hook stuff. and then back in voicemeeter, i just routed that virtual output back into my real output, and adjusted the sample settings so the latency was basically perfect! and i can easily change the volume, too.
third, since my display isn't 1920x1080 (it's 3440x1440, which is just all wrong for sdvx), i'm using lossless scaling (relatively cheap program on steam) to scale the borderless 1080p window to 1440p with some sort of nice upscaling method (similar to dlss or fsr), so my game looks super nice! i was also able to change the vertical position with it too, since my display has extra height on the top and bottom. if your display is a normal 16:9, you could also try using magpie, which is a free open-source program super duper similar to lossless scaling! i just couldn't get positioning to work correctly with it, which is why i abandoned it...
oh and also, the reason i didn't just set the window to be 2560x1440 or whatever is because the scaling was AWFUL!!! every single angle was super duper aliased~ it looked so rough!
these are all of my lil tips and tricks! i hope this helps!!
