Yes you are correct. you would like to use DateDiffWorkdays function to calculate workdays between two dates. You can use default issue property Issue created date as one parameter for this function.
You are also correct, about the usage of measure Transitions from status first date to get a DateTime for a specific status change. You can use it in a tuple with the Transition status dimension and Time default member as well.
Here are some similar topics on this with some formula examples:
The first one might be the most relevant to you, however, check others as well:
The examples contain Transitions to status first dateswitch it to Transitions from status first date. Those two work in a similar way. Address your status within the formulas as well.