Is this the proper base URL (Cloud) to use for a SharpTools dashboard tile to display variable changes taking place in real-time from RM rules that manipulate those hub variables?
No, where did you find that? Maker API does not support hub variables.
A Rule Machine rule can use an HTTP endpoint to set a hub variable, but I can't think of any existing way to get the value directly over a similar endpoint. You'd probably need to create a connector (of whatever type works for your particular application) and then use that device in Maker API -- if you're just looking to display the value. It's not clear if that's what you mean.
I will definitely look into that approach as I'm certainly struggling with Hubitat/SharpTools integration currently. My RM rules are manipulating variables to calculate metrics made from movement & door opening patterns, & its that information that I'm trying to use to create data visualization charts using SharpTools Custom tiles. Not as easy as it sounds.