Did not see your original post - missed it.
Looks like you have got the tent almost finished, so most of your questions you have answered yourself.
Does your tent have an inner like the Soulo?
If so, your fly will just be a 'cap' fly, like the one on the Soulo, and need only a small amount of fabric.
Since you are building a tent for all conditions, it would not make sense to skimp on just a small amount of fabric for a small 'cap' fly that will be exposed to heavy winds, ice etc. Something nylon, around 30-40 denier and 40-50 grams per sq. meter with a hydrostatic head of at least 3000mm would be durable, and would be light, given the small amount of fabric needed.
On the other hand, if there is no inner, and you need to make a full fly, that is another story. Then join the search for the lightest material with acceptable water resistance. There are many different opinions about that. I am guessing you are in Europe, where many good fabrics are available from the German companies.
With good stakes and guylines, the tent looks like it will withstand the weather.
It might be a good idea to be able to zip up the vents a bit under the 'cap' fly, though, in case of monsoon type rain.
It is now a moot issue; but I think that the conventional lap felled seam with the pole sleeves, or clip connecting pieces sewn into the seam as you have it, is as easy to make and strong as any other method. Because the clips do focus the stress over a smaller area than sleeves, some companies reinforce the tent fabric in the vicinity of where the triangular clip connecting pieces are sewn into the canopy seams.
Even though the tent is red, I'm green with envy.