Dutch Flat Mates Double Sandwich Sums: Surely Not Another Variant?

Today's Puzzle:

It may only be me, but I have not seen the double sandwich sums before! It plays fantastically as a fusion with Dutch flat mates.

Link to Play:

Dutch Flat Mates (Double Sandwich Sums) by Flinty https://tinyurl.com/SudokuSleuthDutchFlatDoubleSan

Rules:

1) Normal Sudoku rules apply 2) 5's live in Dutch Flats = All 5's in the grid must have a '1' directly above it or a '9' directly below it. It may have both, but it doesn't need both. 3) Double Sandwich Sums = Digits 1, 5, and 9 are crusts. An outside clue gives the sum of the digits sandwiched between the two closest crusts to the clue in its row or column.