[Lawrie]

Well, I certainly don't want to argue if my suggestion is useful for you or not. That's for you to decide.

But FWIW I just want to mention that I (before I switched to face recognition) for quite some time used one of the bluetooth clickers you can get very cheap from Aliexpress or even Amazon (something like https://de.aliexpress.com/item/1005006996638025.html ) tied it with a rubber band to my left index finger and clicked both buttons for paging forward and back with the left thumb. 

It worked quite well for me as a pianist and I don't see why this shouldn't work even better for your instruments where you certainly don't need the fingers as much as I.

No argument intended mate, just keeping you up to date.

FWIW I just purchased one of those clickers you linked me to, but with my Onyx Boox I haven't had any success at all with the "phone camera remote" style remote.  Not sure why and I've tried several.

I can certainly see how your fingers are quite busy, but surprisingly enough less of a trombonists fingers are available than you might think.  I shall have to experiment some - I suspect I might get away with attaching it to a slide tube, or perhaps one of the slide braces.  Always assuming I can get it to work at all.

Thank you.