My Droner has been unable to get the Adv. Class Subskill. I have all core skills @ 21 with the exception of tractoring. It's been suggested that I do a skill reset... but in addition to the 500m and the cost to relearn all the skills all over again... This alt has Station Mastery 23. Losing that, even for a short period of time, could cause a lot of problems with my bases. Is anyone else having this issue? Does anyone have any suggestions?

You can retrain Station Mastery 23 in less than 5 minutes after resetting. Once you retrain it, your bases will be exactly as they were before the reset.

I'm not sure why you can't get the skill, do you have any of your class subskills to 21? If so, you might be locked out of doing it until you actually reset.

Unless your bases are currently under attack you will most likely notice no effect at all from resetting and retraining StM.