Normally, you would get a long USB cable to plug into your graphics card and your Quest 3
Er, no. You plug the USB cable into the PC's USB 3 port, not the graphics card. You seem to be confusing with the now-defunct Rift S headset.
The link cable also powers your headset, so you aren't draining the battery while its plugged in, so if you want to do your 8+ hour marathon sessions in VR, nothing will stop you mid-session
Hardly. USB ports on PCs don't have the wattage to keep the Quest headset fully charged compared with the official wall charger. The battery will drain a little slower (maybe lasting up to 3 hours), but it will still drain. There are third-party solutions that use a y-splitter to connect both to a wall charger and a USB port at the same time, but those solutions can have their own issues as well, as power is then being supplied by two separate sources which can cause hardware issues.
and that your GPU has the necessary mini USB port
The Link cable is USB-C 3.0. Most GPUs use the Display Port connection and that's only relevant to the obsolete Rift S headset.