Flysky throttle setting question

Probably not relevent - but on my older Futaba's I used to set a good idle speed with the stick down and the trim up. Mechanically adjust linkages and stop-screws for trim up stick up for full throttle, and trim down stick down for cut-off.
These days, of course, being all - electric, that is totally irrelevent, and with modern radio, you can reset the servo centre to be anywhere you want with the trim slider/blipper in the middle!

Throttle-hold - is intended for helicopter aerobatics - maintains high rpm's, whilst allowing the pitch of the blades to be varied (via the same throttle stick). It might serve as an idle setting.

Upgrade? to a Spekky DX6 - I would have thought that was a DOWNgrade - (stirs pot and beats hasty retreat!)

Try a squirt of Servisol switch cleaner - NOT WD40 - on the dodgy ones, it is probably just muck that has found it's way in.
This is a problem that oily motor enthusiasts have, especially if they wash down their TX as well, after a session, to get all the grease and fingerprints (and blood?!) off.