Logic Masters Deutschland e.V.

Previous Dutch Flat Mate Puzzles
This has been a weekly series since '18 Oct 23'. Here is a spreadsheet link for all the previous puzzles in the series, which also has a few teasers for upcoming constraints/guest setters in the series (subject to change).
This alternative link offers a limited LMD search for Dutch Flat Mates.

Special Thanks
Here is the original Dutch Sums that I made, which was based on an idea by SennyK.
I feel Flat Mates add a lot of value to this constraint.

To play the puzzle, please click the image:

Initial Setter Difficulty: 2.5 - 3 / 5

Normal Sudoku rules apply
Place the digits 1-9 so that they do not repeat in any row, column, or 3x3 box.

Dutch Flats
Every 5 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.

Dutch Sums
A clue outside the grid gives the sum of all continuously adjacent digits with a difference of at least 4 up to (and not including) the first digit with a difference of 3 or less in that row or column from the direction of the clue. A digit in the first cell is always included in the sum.
Example: A row of 192876543 would have a sum of 1+9+2+8=20 (from the left!), as 8->7 breaks the chain.

Hint to start

Consider the outties in C5, and the impact of Dutch Flat Mates would have on 'Happy 5s' overshooting the sum in some circumstances. How many low(ish) digits remain in C5? Avoiding overshooting the sum is a necessary consideration for the puzzle as a whole.

Nudge for a step shortly after

C5 can be fully completed with just those two clues and the rules.
The next deductions are in Row 8 with a potentially tricky step being that 1 and 5 can't share a box, or the 5 in Row 9 would be sad.
It's also worth double, triple, checking that you aren't overshooting any sums.

Visual Rules Example:
Traditional Dutch Whisper style lines added to show which digits are included in the sum, with the assumption of prior knowledge in the hope that it aides understanding.