I started my seeds in the greenhouse this year but due to very hot 2 weeks in march, they all got cooked, even the soil was moist.
The germination ratio was very poor, once moved outside, some of them germinated, but not the way i wanted.
My problem is i need to keep door and window open so it doesnt get too hot, i try to keep temperature around 30 Celsius, however with that temp, humid is very low, maybe 30% (otherwise i will have 50degrees or more - same as you)
I had to move all seedlings out now, only tomatoes and other veg are left, also some tropical plants and couple of small ones that seems to like hot,
Larch seems to be loving it, some prunuses also, albizia, baobab, fuchsia, cotonaster, all are fine, maples not doing well, unfortunately Azalia got toasted as well
Vent will help you, however until you have door open and bit of draft, it will still keep very warm inside, problem is humidity that is going low
I installed shading net yesterday on the side when midday sun shines the most, i will see how it goes, its cloudy today so cant say