Dynamic comparison of 2 tables

try this

let
  OriginalB = Table.Buffer(Original),
  UpdatedB = Table.Buffer(Updated),
  Source = Table.NestedJoin(OriginalB, {"Key"}, UpdatedB, {"Key"}, "Updated", JoinKind.Inner), 
  #"Added Custom" = Table.AddColumn(
    Source, 
    "Changes", 
    (k) =>
      List.Transform(
        List.Skip(Table.ColumnNames(OriginalB), 1), 
        each 
          let
            orig = Record.Field(Table.SelectRows(OriginalB, each [Key] = k[Key]){0}, _), 
            chg  = Record.Field(Table.SelectRows(UpdatedB, each [Key] = k[Key]){0}, _)
          in
            if orig = chg then null else _ & " : " & orig & " -> " & chg
      )
  ), 
  #"Expanded Changes" = Table.ExpandListColumn(#"Added Custom", "Changes"), 
  #"Filtered Rows" = Table.SelectRows(#"Expanded Changes", each ([Changes] <> null))
in
  #"Filtered Rows"

and maybe also consider a Table.AddKey on the [Key] column.

It will also be worth leafing through this series of articles : Chris Webb's BI Blog: Optimising The Performance Of Power Query Merges In Power BI, Part 1: Removing Columns (crossjoin.co.uk)

There are better tools for this - for example Alteryx or RapidMiner or Knime.