The conception of expression, formula, or whatever they called, is a general CAD feature in many CADs. If someone use calculators like excel or have some (even small) programming experience, expressions is almost self explaining. What is not self explaining is, what can be accessed. For that is try and error and the wiki. But when you know a part of it, then you figure out the rest almost complete.
That is up to you. But to be brutally honest, if you have no CAD experience, and have really nothing with sheet metal (or generally with machining of steel) till yet, creating a full parametric sheet metal CAD, which is also produceable, is a damn hard task. If you are a hobby technical enthusiast, then maybe yes, but you seems to underestimate the complexity, in CAD and machining/producing, even on a very simple part like this.
And general according CAD, i sound like a broken record, but CAD is no fast just plaything, just because it is (an easy accessible in the case of FreeCAD) program. I always compare it with machines, like a lathe or milling machine. Even they conception of the machines are similar or mostly the same, you must know the machine specific and environment, else you only produce cull. Example a fit 100 +/- 0.02 mm, if write a CNC program (or do it manually) and you position the machine in that coordinate and make the fit, will not be in most cases 100 +/- 0.02 mm , because of the stiffness the machine, scale arrangement, cutting edge, material, temperature, infeed, axis arrangement .... . You must know many details, and that case is also in CAD. And like machines, even brand new, have quirks, CAD also have them.
Greetings
user1234