Hello, Tundi, and welcome to the Grammar Exchange.
"weekday engineer" is fine to refer concisely to a person working as an engineer only on weekdays. Here you can see an example in context.
This reminds me of the time when, being young and having a regular job during business hours, I would be called the "overnight translator" when I engaged in translation projects after work.
If it were not a tattoo, a comma would be suitable, as shown here:
Others prefer a period, a hyphen or a slash, but I find the comma to be superior from a linguistic point of view: