Glad to hear you're nearly there.
Plug looks good, but to be honest I only use the plug colour to check main jet size after completing a plug chop.
I can only assume the burnt crusty bit is something that's got into the combustion chamber, maybe when you've had the carb apart, I really wouldn't worry about it.
Do you do much wide open throttle riding? I'd so I would try a bigger main jet, just to be safe. I started with a big main jet, that was obviously too rich then came down one size at a time until it pulled WOT cleanly.
It's safer to start big on the main jet then move smaller than other way round.
All the best, Dave.