A few months ago I messaged a former colleague, "Hey, do you remember
how to make variables dependent on each other?" It might have been her
asking me. In any case, I was looking for a way to do this that wasn't a
client script which would build these dependencies.

Eventually, I found it. For lookup select boxes and lookup multiple
choices there is an attribute available called ref_qual_elements that
you can set with a comma-separated list of other variables. So what
happens when you change the value of one of the listed variables is a
server call is made and the options are re-evaluated and unset.

This is great if those options are pretty static. If, however, you let
users disable options, items that use these variables that are in
progress won't display these option.

Here's the official documentation about this variable
attribute
.
It doesn't describe it as a way to save time, it will have to do.