I think that’s a good approach - start with the basics and build up from there. 
I would probably start with just controlling the code on a single lock. To be perfectly transparent, you’re jumping into some of the more intermediate-advanced features by using variables and connecting them with dashboards, etc.
I played with a really primitive version of this when someone else had asked the question a few months back:
In that dashboard, I have:
- Label Tile (Lock Code Manager)
- Variable Tile ($LockCode → Tap to enter Lock Code)
- Rule Tile (Save)
The overall concept was I created a variable named $LockCode, then added that to the dashboard. Rather than just have the rule automatically run when the value of the $LockCode variable was changed, I created a rule with no triggers and a command to set the lock code. Then I added that rule to the dashboard so anytime that rule tile was tapped, it would run the rule.
In the rule, the main thing was a Device Command that set the lock code (slot 2, always named “Custom User Code”, but with the code being dynamically set from the $LockCode variable). I don’t have a copy of that rule anymore, but I suspect it looked something like the following:
The last 3 actions in that rule are just to have some visual feedback on the dashboard. That way I could briefly see a message that the code was saved and then it would reset to blank.
The commands tend to be documented in the SmartThings Capabilities documentation.
As a side note, the LUM app from RBoy is a powerful app with a number of features. You’re not going to get a ‘plug and play’ version of that here as rather than being provided with a purpose-built app, you’re given a set of tools with the Rule Engine from which you can build whatever logic you desire. Have you reached out to RBoy about their transition plans for LUM?