I am looking for a good, stable and cheap graphics tablet for online math education that you use to use and love to use. I need to prepare remote videos.
My recommendation for a usable, inexpensive drawing tablet would be the XP-Pen StarG40. It's what I've been using this year (in conjunction with Blackboard/Collaborate) and costs about $40 as of this writing. It's the lowest-cost tablet I've seen, and it works reasonably well (there are encoding issues in Collaborate that make it always a bit jittery, but it's perfectly smooth in MS Paint, for example).
At the start of 2020, I got this, and also a somewhat higher-end model from the same brand for my graphical artist partner. To our surprise, the lower-end model actually works quite a bit better. She's used much higher-end Wacom models in the past and certainly think that those give a better experience (at much higher cost). On my recommendation, we got these XP-Pen models for everyone in our mathematics department.