Hey Oliver, this is a tough call. I would hold out for the promotion and vesting if it’s only a couple years. Personally, I did a MS in CS/ML part time while working. It was a little brutal at times, took almost 4 years, and limited my choice of schools. But it was very good for my career, since I didn’t have to sacrifice any job progression time. It’s not the right choice for everyone (I don’t have kids, so more flexibility), but it’s one option.
I will say that while a year or two to wait for a promotion/vesting might seem like a lot, it will fly by, and you will be in a very strong position from there. If you are donating and enjoying your job in the meantime, all the better!
If possible, I wonder if you could negotiate a 1.5-2 year break from your company to go for the MS, pause the vesting (rather than lose it), and come back in at the senior position. This would be the best of both worlds if you could swing it, I’ve definitely heard stories like that. If you get into a top school, you could have good leverage because it looks good for your company. I’ve even known people to swing the deal so that the company pays the tuition with a 2 year commitment after. If after a year you love research, back out of the deal and continue for the PhD.
Sounds like you are in a good position right now for all the reasons you state, so be careful before throwing that away, you could always get unlucky and find yourself simply starting over at a new company after getting the degree.
Good luck!
Oh, and regarding the degree itself… I liked my MS CS program for the most part. The cost of tuition is becoming a tougher sell with all the cheap options to learn online. Hour-for-hour, you could probably get a better education by reading articles and practicing skills, because you can learn exactly what you want, and classes are not quite as cutting edge as podcasts/articles/meetups/etc.. For example my neural nets class taught theory but no practical skills like tensorflow/keras/pytorch, which really annoyed me. But the structure does help you stay focused and organized for multiple years. Mine was online, so I can’t speak to the value of networking. The value of the degree on the resume is definitely real, but only a little better than 2 years of experience, and probably not much better than a promotion. I can’t speak directly to a PhD, but I was on the fence, skipped it, and definitely do not regret my decision.