This is kinda like trying to smell rotten eggs over the telephone, but here goes:
If the bike is 'all stock'.
These bikes are known to run very lean, and therefor pretty hot. I've seen more than a few blue their head pipes back to the muffler. To make the pollution gods happy, the bikes are jetted lean in the lower ranges, which is where most folks ride.
I know everyone has their own idea of a good time, but I would never run 5-40 oil in an air-cooled bike unless I was living at the North Pole.
A combination of thin oil and a hot running air-cooled motor means the oil thins out and is more likely to sneak past the rings.
Which makes me think that the carbon is oil, not fuel. If, as you said the bike is stock, and has not been rejected way too rich.
The combination of the clean porcelain on the plug and the build up of crap on the side electrode makes me think that the bike is running hot (as stockers do) and is burning oil.
How was oil consumption?

Lower is thinner. So, if the 5-40 is sneaking past the rings, the 5-30 would be blasting past them. In my bikes I run 10-40 in the winter and 15-50 or 20-50 in the summer.
I know there are some folks on the forum with very different ideas, and I respect them, just saying what I run.

Arman, I think you may be on to something. What bothers me ,is the jetting, a 55 idle jet may be over the top (rich) at his 200 ft. ele.The 145 main may be running hot(lean) for anything above idle,and he made no mention of a spacer mod. 5w-40 could thin to 3-4w in a hot engine,and flow by the rings.Rotella (dino) 15w -40 might be a better choise for oil,and you really need to rejet the carb,a 3/4 -1/2 spacer mod,and a 150 -152.5 main at least.

Jwenrich, was your header pipe turning blue? that could be an indication of either to lean (usual) or to rich(rarely the case).

Should the intake valves be that black (rich) while the exhaust valves are that white (lean)? I know the exhaust runs cleaner due to heat but that looks like a huge temperature swing to me. You're definitely burning too much of something.

Yeah the breather runs to the airbox. Also, upon cracking it open, no mesh was present before the breather tube

Could lack of a mesh filter contribute to this? Do I need a tiny filter thing on the end of the breather hose?

General consensus on the packed mesh is it gives oil vapor something to smash into , condense and drip back to the crankcase. If it's not there, And the vent tube goes to the intake, that sounds like a recipe for carbon in the cylinder.
And oil consumption.
I wonder if a scrubby pad for pots would shove in there..