Personally, I would pin my hope on the new router / ISP having fixed that old lag spike issue. If that turns out to be the case, it was an issue outside of your PC, or at the very least, an issue with your PC’s WiFi adaptor communication with that specific router.
Hopefully the AT&T router / connection does not have these issues, and hopefully they’re able to resolve the new issue.
The AT&T router IS the new router, the router with which had the titular “SLOW/inconsistent(?)” Internet Speed across applications past 8 pm.
The old router/gateway is from Xfinity, it’s the one that my phone kept connecting to as I mentioned earlier and it’s the one I’m having the lag spike issue with.
Just saying this again to make that clear since from reading your latest reply I’m unsure if you’ve got that.
I hope AT&T is able to resolve the connection issue I’m having with their router too.
I haven’t contacted Xfinity about the lag spikes yet because I’m still unsure if it has more to do with their router or my pc, I’ll let AT&T’s support do their thing first.
Yes, that’s how I understood it 
To rephrase more clearly, hopefully the AT&T router / connection does not have the old issues, and hopefully they’re (AT&T) able to resolve the new issue.
It turns out that this issue I had with the old router was caused by Xfinity REALLY not liking MAC Address Randomization. Each time a new address was generated, this old router would throw a fit and kick me out since it didn’t recognize that as my PC. The solution for this router specifically (an XB8) was to create (or edit if it already exists) the following file in /etc/NetworkManager/conf.d/ and restart NetworkManager:
wifi_rand_mac.conf
[device-mac-randomization]
# disables random mac address for scanning
wifi.scan-rand-mac-address=no
[connection-mac-randomization]
# generates a random address only once per ethernet connection
ethernet.cloned-mac-address=stable
# generates a random address only once per wifi connection
wifi.cloned-mac-address=stable
This did not solve the issue I have with the current (new) AT&T router’s slow speeds past 7-8 PM however, and such could’ve been gleamed from the fact that MAC Addresses aren’t randomized on Windows by default.
Help from AT&T on that matter is delayed for a few days because of harsh weather coming soon, so I’ll need to play the waiting game.
As I’ve said already, I’ll keep you updated.