I need help modifying an existing DAX formula

// One way of doing this is:

Media Due =
var MonthNumber = dimstartdate[monthnumber]
var YearNumber = dimstartdate[YearNumber]
var MediaType = T[Media Type] // adjust this to your model!
var Output = 
    if( YearNumber = 2023,
        switch(
            MediaType,
            
            "Digital",
                switch(
                    MonthNumber,
                    1, "12/14/2022",
                    2, "1/19/2023",
                    3, "2/15/2023",
                    4, "3/23/2023",
                    5, "4/20/2023",
                    6, "5/17/2023",
                    7, "6/21/2023",
                    8, "7/20/2023",
                    9, "8/23/2023",
                    10, "9/20/2023",
                    11, "10/18/2023",
                    12, "11/21/2023"
                ),
            
            "Direct Mail",
                switch(
                    ... -- similar to above
                )
    )
return
    Output
    
    
// ... but it'd be even better to create a driving table
// that would store a mapping between Year, MediaType,
// MonthNumber and the output values. This then could be 
// stored outside the report in some source system and 
// adjusted without even having to touch the DAX.
// The above solution is the first naive one but I encourage
// you to create the more robust one with the driving table.