I am sorry to say that the exact model you describe doesn't exist. I mean, I don't want to be negative immediately at start, but I need to adjust your expectations a bit here - there is not a stroller, and a double stroller even less so, the hat would be lightweight and compact and upright and for that price and with all the details :-D. With double strollers, this is even worse - they are naturally heavier, larger (bulkier), and the less equipped the cheaper they come.
The tandems are even worse weight- and fold-wise, so a side-by-side is still your best bet.
I would normally say that the priority here is that your eldest would last in the pram meaning a SPACIOUS SEAT is a priority. A spacious seat however means a larger stroller and other disadvantages, and you have such a seat already but the stroller is too heavy for you.
With the low weight, that's even harder, as even the light ones are usually 11 to 13 kg which is close to what you have already - and you say it's too much. And the lighter ones are always very rattly and very small in the seat part (like a Joie Aire Twin or the Baby Monsters Kuki Twin), so no way you'd fit your older one in there much longer.
My advice here? The best-fitting model seems to be the valco Snap Duo. It is the closest to what you need - not too heavy, rather upright (not a 90-degree angle but quite upright actually if you pull the reclining strap as far up as possible), lies flat, with two large canopies... if you cannot find a cheap one, try looking for it in classifieds - pre-owned is also a solution. The largest problem there is perhaps the size when folded - it is good for a double, but still, it is not small. It is smaller hat the Thule Glide Double you own right now, that's for sure.