Been 2 weeks with no input lag or floaty mouse. Mouse hasn't felt this good since 2014. When shaking my mouse all of the small mouse jitters are being picked up instead of that weird "smoothing" floaty sensation that comes back a day after reformatting (i reformatted my pc probably like 30 times at this point, and built 4 custom pcs trying to fix this issue

Crossing my fingers that it doesn't return, and after 2 weeks there's been no sign of it coming back. I will update the thread if it returns.
What triggered this change:
1. Reset all Steam notification settings to default from the steam website: Steam Account Details > Notification Settings > Expand ALL the options and match each tickbox to a brand new steam account's notification settings. Before this I had several mobile app push notifications and steam client toast notifications checked that were different to default. I have NO idea when I changed these but something triggered the change because my mouse movement was responsive and snappy before around 2015. Do this for every single steam account you've logged into on that pc.
edit: default settings I used as taken from my alt account, requested by someone: https://imgur.com/a/Ce4c5wU
2. On discord mobile app, disabled "get notifications within Discord" under notification settings. For some reason when it's enabled, I feel a bit of server lag and weird hit registration. Everything else in my discord settings is default.
I have no special PC tweaks. Bios settings are default + XMP. Nvidia settings are high performance slider setting with power usage set to prefer max performance. That's it. I refuse to make further tweaks, most pros play with just these changes alone and it's good enough for them; I wanted to find the outside cause for my lag. My lag was following me around from house to house and build to build so my thought process was that it had to be an external factor.
I'm wondering if the cause is due to the steam and discord PC / Mobile apps pinging each other that's affecting mouse lag/hit reg. Couldn't tell you to be honest, but these work for me. Also the lag can be replicated again by reversing any of the changes I mentioned above strangely.