You just threw me back into time I spent on EKF implementations on early iPads over a decade ago (as an exchange student), as part of a M.Sc. in Control Theory... before I had learnt about KF. The goal was to compensate for the drift observed on the position of those devices which made them useless for the need (art exhibition stuff).

I don't think I even fully completed the project because my understanding of both quaternions and KF were at that point pretty shaky, this could be a fun project to pull with some old hardware now..