Sliders work similarly to labels in that you bind them to a shared variable, you can then simply access the shared variable in your scripts to get or set the value. Shared variables (unlike normal variables) are not attached to the macro trigger that owns the macro, they are shared amongst all scripts. To make a shared variable, prepend an at symbol "@" to the variable name. In order to avoid breaking other scripts with local variables, shared variables exist independently of local variables so for example @#foo is a shared counter variable and #foo is a local counter variable.
So to use the slider value, simply enter a shared variable name eg. @#myslider and you can then use this in your scripts.
GETPROPERTY and SETPROPERTY are mainly for runtime customisation of controls (eg. changing colours, labels, etc) and are not super efficient, this is why labels and sliders use the concept of binding to a variable since it's a much more efficient read/write operation and can support many scripts reading and writing the value at once with negligible overhead.