3x1.5 or 5x1.5 NYM in the roof box for LED spots?

Maybe you want to connect a motion detector or dusk switch, that's why take 5

too much never hurts, but what are you planning to do with the five wires?

The idea from to use 2.5 quad wire is not so far-fetched.
A disconnection of the fuse in case of a short circuit within a specified time must be ensured.
With the length of the line, the resistance also increases, which consists once of the length from the last consumer to the fuse plus the network impedance. This is then called the loop resistance together.
This can then be determined with a suitable measuring device.
Therefore, make sure that your electrician creates a measurement protocol of the system.
If there has been a fire, the newspaper will say: Fire caused by short circuit, surely everyone has heard that already.
That is then one of the causes.

And which cable do I use for the connection to the GU10 socket? In a video, I saw someone using the rather rigid NYM cables only for the supply line and then a flexible cable to connect the GU10 sockets.