Best solution small flat roof replacement

[Tigercubrider]

My house has a 20 year old (plus) flat roof on a rear extension. At a guess 3-5 M square.
The extension is about 40 years old and from memory only has the yellow cheap fibreglass in it.
The extension fills in the space between a shorter garage and a longer house.
(two original walls and two newer walls)
The rear of the house faces south

On the two original house walls, are two windows that overlook the flat roof.
One is a fixed window, one could be used as an exit onto the roof in an emergency
The windows are at least 300mm above the roof.

I am assuming that the felt roof will be on it’s last legs soon.
So the question is …
If replacing what with?
Warm or cold roof?
What material?
How much insulation?
if the existing sheet material (ply/chipboard) is in good condition would it be left in place?
Best to strip it off and replace it anyway?

One option could be to fit a sloping tiled roof falling to one side (same side as the existing)
with the proviso that it is below the window.
I can sort out more accurate dimensions but the fall is not massive for a tiled roof

Personally I suspect that tiles are expensive and probably not practical?
However probably last longer than felt/rubber/fibreglass.

thanks