My problem with flatpack is that it is never exactly right for the location (but then every house I have lived in has been over 75 years old, so maybe more recent houses have more "standard" sizes) and so every installation looks like it has been made to fit rather than fitted. Then there is the question of whether the style and finish is what you want.
The other side of the coin is that weekend warriors often start a job like this by removing the existing kitchen. I saw my Dad do this and the pickle he got into because Mum didn't have anywhere to cook and the new kitchen was "just coming together" in the shed. Domestic bliss, NOT! Get the new kitchen ready first.
My take on it is - have a go at it yourself.
Measure everything in your kitchen (especially check how far the walls are out of square) and note where powerpoints and water are (or need to move to).
If you can be bothered, prove your concept by making a model (accurate to sizes, but it doesn't need to be more than that).
Make it all up in units in your shed and store the completed parts.
If you are replacing appliances, order them and
WHEN EVERYTHING IS READY TAKE TIME OFF WORK AND INSTALL.