I can't for the life of me figure out why I'm having such terrible network issues lately. It's only when I'm streaming too. I don't have any issues when watching Twitch or Youtube, Netflix etc.
Basic networking observation - what you mention (watching anything, Netflix, etc) is ALL download traffic, and they all build in buffers to handle network traffic jitter. The Internet wasn't desired initially for jitter/latency sensitive traffic. Many band-aids have been added on, but basic design and assumptions of TCP/IP is packets arrive whenever, in whatever order, and all will be well. The fact that it works for voice etc is a marvel of modern engineering... but, you are still swimming upstream (pun intended)
Now, as an IT person, and when dealing with gaming PCs in general, typically Game code quality is crap, and gamers rebuild their OS regularly because of how bad games and their own 'tweaking' ruin the OS. So, in general, I laugh at the thought of trusting a gaming computer to be operating well... could be... never something I'd count upon. ever. That a gamer doesn't notice how bad things are doesn't mean there aren't problems (EVERY gamer system I've ever checked has been an absolute disaster, security is effectively disabled, etc)
I mention the follwoing only as you indicated replacing a lot of Internet related hardware. A point-in-time test that would be fairly reliable would be a clean OS build, all excess start-up apps disabled, no 3rd party A/V, etc and just OBS (no plugins, etc). Plug straight into Internet modem and stream static image at desired bitrate ... do you have issues or not? If you don't have issue, then start plugging in rest of networking gear (router, etc). test again (with NOTHING else (phone, TVs, smart devices, etc) connected). If all is well, then you you know that it isn't the network, or where to start looking at least
ISPs don't usually test network throughput, especially not for upload jitter, ie game streaming (Advanced network admins at the ISP could, but your field techs, even Level 1/2 engineering support... probably not, or at least nt adequately). However, similar protocols, network implications when doing video conferencing, though usually at much lower bitrates
Then (having not looked at your OBS logs as more knowledgeable folks already have) it sounds like you need to clean up you OBS setup (maybe wipe and start over?), sticking to quality code elements only. I know that isn't what you want to hear. but if your focus is on fixing issue, this is how to be sure of where your issue is coming from
Good Luck
