Matrix with dynamic values

1) Create a disconnected table for the matrix columns

Matrix Columns =
VAR FixedCols =
    DATATABLE (
        "ColType", STRING,
        "Label", STRING,
        "MonthStart", DATE,
        {
            { "CURRENT", "Current Stock", BLANK() },
            { "SAFETY",  "Safety Stock",  BLANK() }
        }
    )
VAR MonthCols =
    SELECTCOLUMNS (
        VALUES ( 'Date'[MonthStart] ),          -- use your month start/date key
        "ColType", "MONTH",
        "Label", FORMAT ( 'Date'[MonthStart], "yyyy-MM" ),
        "MonthStart", 'Date'[MonthStart]
    )
RETURN
    UNION ( FixedCols, MonthCols )

Then sort Matrix Columns[Label] by Matrix Columns[MonthStart] (and keep fixed ones first with an additional sort column if needed).

2) Use THIS in the Matrix

• Rows: Region, Country, Material (your dims)

• Columns: Matrix Columns[Label]

• Values: a single measure like below

3) One measure that switches logic per column

Matrix Stock Value =
VAR _type = SELECTEDVALUE ( 'Matrix Columns'[ColType] )
VAR _month = SELECTEDVALUE ( 'Matrix Columns'[MonthStart] )
RETURN
SWITCH (
    _type,
    "CURRENT",
        CALCULATE ( [Current Stock], REMOVEFILTERS ( 'Date' ) ),
    "SAFETY",
        CALCULATE ( [Safety Stock], REMOVEFILTERS ( 'Date' ) ),
    "MONTH",
        CALCULATE (
            [Available Stock],
            TREATAS ( { _month }, 'Date'[MonthStart] )
        )
)