Sorry, but how will measuring help? Won't. All it will do is quantify and confirm what you already know. If the wow seems related to platter rotation that points towards the bearing. Which you would want to clean and lube anyway as that is the next logical thing after the belt, which you already did.
If when changing the belt you also cleaned it and everything it touches with alcohol, that eliminates the belt.
We are now down to the motor. Look closely, is it perfectly straight or bent at all? Listen close, is there any sound with the same pattern as the wow? Can you lube the motor at all? Use only the tiniest amount of oil if you can.
My guess is "slight, quick, pitch changes" could be the bearing. It is an older table and could be going dry. Inspect, clean, and lube. I like Liquid Moly 90wt gear oil but you could use anything and Origin Live is the creme de la creme. In any case do all the steps above. Really just smart ongoing tune-up/maintenance. Can't hurt.