I've tried vinegar and wd40 and would really appreciate any helping hand for this noob here!
Heat can do wonders here, even at just boiling water temps. The housing will expand more than the cartridge threads, possibly breaking it loose but more likely just easing friction.
Either drizzle boiling water on it or heat it with a heat gun, hair dryer, or plumber's torch. Obviously be careful with the latter to not scorch the finish or any nearby objects.
I've often found that PB Blaster™ will work where WD-40™ will not. Also a pair of locking pliers (like Vice-Grip™) clamped down really hard might get you a grip where all those corners have been rounded off.
You may end up having to replace parts because the serrated jaws of the pliers will tear that up. Of course, as rounded as those corners are, you'll
probably want to replace that anyway. If you try to reinstall that, you'll end up not getting it tight enough because of the rounded corners, or, if you do, you'll have this struggle getting it off next time.
At some point it is not worth it and get a new faucet. That being said, for a Moen shower/tub valve, I had to drill a 5/16 hole in the back, put a 1/4 punch through the hole and hit it with a hammer to remove the old cartridge. Then ground the back of the valve flat and soldered a copper patch on it. Worked fine for a few years until I moved.