@ejr
Erin, not sure if this will help or not but you may be able to adapt to what you want. I have a serial multizone amp that does not have an increment command for volume. All the volume commands have to be sent individually for each level requested. The following link (which I have yet to adapt but think I can make it work) may help you to adapt a slider to a current single “switch” command. The magic is in the rule.
Another consideration may be to change from a switch to a contact.
Hope some of this is helpful. Let us know if you have any success.