You lose more heat through ceilings than through other surfaces (because the warm air is touching it, and because of good convection currents).
The insulation value of glass is very low. To put it in USA terms, a single pane of glass is R-1, while many house walls are R-20 and ceilings R-30. I've seen superinsulated roofs at R-60. Even triple-pane glass is only R-3; inert gases inside help, but over time they leak; you probably can't have a good seal if you want to run a curtain through them.
During the day, insolation will warm your house through the ceiling (probably too much, even in your part of the world), but at night when you're only losing heat, you'll really feel it. You'll use a lot of energy to stay warm.
At the same time, it would be very expensive. Windows are generally an expensive feature in a house. Sloped windows should be tempered for safety, which makes them even more expensive. And you're talking about a huge area.
Roofs in particular get beat up pretty hard; standard roofs (asphalt shingles) around here last about 20 years.
Meanwhile, you lose the opportunity to put photo-voltaic panels on your roof.
However, you can get a lot of what you're looking for with a much less ambitious approach.
Small solar tubes bring in a huge amount of light for their size.
A skylight over your bed can let you stargaze in comfort. Put your bed up high for a wide viewing angle.
Spend more time outdoors.
The best and easiest solution is unfortunately no longer possible without removing and reinstalling the metal roofing: you vent above the sheathing and below the metal roof. The way you would do this is by installing the metal roof on vertical eave-to-ridge battens, with a metal ridge vent piece at the ridge. This way, all the ventilation is on the outside and you don't have to worry about complicated interactions with your finished attic.
Best Answer
You wear a harness attached to a rope that is thrown over the roof and tied to something solid such as a tree.
The harness kit may look like this:
The harness is most often used in a manner as shown here. A short connecting rope goes from the middle of the high back over to a D-Clip that then attaches to the main safety rope via a Robe Grab.
Rope grabs come in various styles (one shown below) but are basically designed to permit them to slide along the rope when they are directly pushed but will clamp tightly on the rope when pull tension is applied to the connecting ring from the D-Clip and short rope to the roofer.
This picture shows a harness in use on a roof....albeit not quite as steep as the OP has asked about.
Here is another in use application.