It's not an easy solder job and they are picky about what NANDs they work with and how they're configured. It's better than soldered to the board but not by much.
Given the price Apple charges for the high end upgrades, it's worth your time to buy the base model, do the uograde, and sell the upgraded unit.
For instance, here's a Mac Studio being upgraded to 8 TB.
https://www.youtube.com/watch?v=HDFCurB3-0Q
