I only get a fraction of the bandwidth that other users get.

WOW, after reinstalling Windows this is the result: SpeedOf.Me Test Result

187Mbps, from my former 15Mbps.

Yeah, something was definitely messed-up in the deep settings somewhere. I'm not gonna toy with the privacy tools anymore, I'll just disable the issues in the Privacy control panel in Windows and call it a day. To hell with my NSA-phobic tinfoil stuff, I'll just do everything I'm sensitive about on Linux (Qubes) instead and use this as my "playing" computer.

Disabling the privacy issues in the control panel, blocking Telemetry via HOSTS + Firewall, and using a VPN should be good enough. The rest of the modifications were probably going too far with it and started causing stability issues.

I've pretty much got Windows reinstalled now and updated all drivers and put in fresh software etc. Guess I'm ready to go now. No idea what the source of the issue was, or if it was even just one thing and not a collection of things that were causing it.

And yeah, you're right, I've been holding off on getting a new computer for way too long. I'm very due for it, I just hold off because I hate parting with my money.

Last update: After doing some fine-tuning, my final result: Speedtest Screenshot

And it's close to peak-hour right now, might be even better at low traffic times. Insane how much of a difference this is.

Welp, thanks for trying to help everyone.