how to dynamically replace values for multiple columns

You can do this using the Table.ReplaceValue function with a custom replace function.

First as others have recommended,  create a Mapping Table.

For example:

You can then use this code:

let

//Replace next line with your actual data source of the splitted table
    Source = Table,

//create list of columns to evaluate
    Cols = List.Select(Table.ColumnNames(Source), each Text.StartsWith(_, "mt")),

//Lookup the Key
//Assumes Mapping Table is a two column table the columns named "State" and "Key"
    Mapping = List.Buffer(Table.ToRecords(#"Mapping Table")),

//Execute the replacements
    #"Replace with State" = Table.ReplaceValue(
        Source,
        null,
        Mapping,
        (x,y,z)=>if x=null then null else List.Select(z, each [Key]=Number.From(x)){0}[State] ,
        Cols)
in
    #"Replace with State"

With an initial "split" table that looks like:

After running the code: