I promised I'd be back, so here are some pics of LatencyMon with everything (seemingly) sorted out:
In order to reduce the hard pagefaults I was getting before, I ended up having to replace Windows Defender. Avira seems to be working really well for me so far.
I also replaced my SteelSeries Apex keyboard with a CM Storm Quick Fire Rapid mechanical keyboard, and I'm running it in PS/2 mode to free up USB ports. It's my first mechanical keyboard, and it's fucking awesome. I got the Cherry MX Red switch version. Love it.
However, I'm not able to run the USB card I bought in USB 3.1 or 2.0. I was kind of expecting that 3.1 wouldn't work with this motherboard, as Asus even states that the card needs to be used with certain Asus motherboards for full compatibility, but I thought that at least 2.0 would work. Nope. The only way the card works is if it's run in USB 1.1, and that's what my motherboard USB controllers were running in. The bottom line is that CSGO still feels like shit. I'm going to get a cheap USB 2.0 card to see if that improves things. Perhaps higher polling rates are more stable with USB 2.0? I don't know, but it's worth a shot.
I'm out of ideas as to what to do about how CSGO feels. Input lag seems to come and go, and it's mystifying.
EDIT: Mouse Rate Checker indicates that 1000 Hz is stable. I'm currently experimenting with some CSGO settings. I was using raw input 0 because I read that its implementation in CSGO is off (apparently, a slight delay is produced somewhere along the line when using it), but I turned it back on. I also put -useforcedmparms -noforcedmaccel -noforcedmspd in my launch options. I'm not sure if these still work. I was under the impression that they don't, but I'm trying them anyway. With both of these changes, mouse movement seems to have improved. It feels smoother and more consistent, although I can't say for sure yet. It could just be placebo. I'll continue to play around with these settings and see how things go.