Hi there, I'm a PD40 user as well, I'm glad to read you DonO
I'm not a very experienced musician though, I played brass for about 6 years before getting to the HD since 2 years now.
I really like this instrument and I practice a lot on the PD40, something like 2 hours every day minimum. I've a very spontaneous approach to music and my first objective with the PD40 has been to "make it sound well enough" to be able to go out and busk with it.
This is quite achieved and these recent month I've spent some time to create some exercises to improve my technique with the instrument. I'm caring a lot more about my posture also but I guess I won't have to mention that to a retired music teacher.
Theses exercises have different objectives; left / right switching, maintaining bass with right hand and soloing with the left (as a right handed person I'm able to do it the other way), double strike with left hand, triple strike with both, velocity and speed exercises, precision exercises, harmonizing exercises, rhythmic exercises, as well as pattern switching exercises.
I'd like to help you and share with you what I know if you're interested but I don't really know how to do it (quickly at least). I guess shooting little videos would be the most effective way to show what I came up with but I lent my camera to a friend at the moment and I'm also very busy these weeks. I guess if you're a bit patient I could shoot these videos during the next weeks / months...
Until then I can share with you some educational materials from other instruments I found useful and inspiring recently.
Mimicking the paradiddles, flams and so on, from the snare can be interesting rhythmically speaking
Although the PD40 got a big difference with the cimbalom (bass bridges are on the sides and thus unplayable with one hand only), the exercises presented here by Kalman Balogh (great musician btw) are quite reproducible with the PD40. There are a few other videos that can be interesting on this channel (Videos named "cimbalom polka", "ejercicio para trasponer a todos los tonos" and "Improvisación-creación sobre ejercicios en La Menor" are the other 3 most interesting as far as I've seen).
Of course everything that is showed in those videos has to be adapted to the specificity of the PD40 but since the cimbalom is also a chromatic instrument I've found some inspiration in those videos. I hope it will inspire you as well.
Btw I'm from Belgium, Europe. If by any chance you are around in the coming times, do not hesitate to contact me.