Hey @emozio! Thanks for reaching out.
I find the "Watched inputs" option to generally be in an odd place, as it does literally nothing if the query's "Run behavior" is set to Automatic. It should really only be enabled if the behavior is set to Manual, as then it has some actual utility. I can think of a couple ways to integrate this existing functionality with the idea that you've proposed, but I'll pass on your feedback and let the Product team take it from there!
For what it's worth, the workaround that I typically use is very similar! I'll build a dummy query - usually of type "Query JSON with SQL" - that references the inputs I want to listen to and then add my actual query as a success callback.
Thanks again for the feedback! I'll provide an update here as soon as I can.