hello, I see these air intakes on the trunk of the roller shutter.
You could take inspiration from this video to improve a little:
I did the same thing in a rental unit, because the tenants complained that they were cold.
You could do something long enough and put two baffles on each side. Try to do this at least 80 cm long.
Remove at least 3 existing holes on each side, and add them in the center or make an opening between 3 or 4 holes in the middle and that's it. The air will pass into the box and will tend to heat up a bit. (this while waiting for better).
*** The air would have to come out behind the curtain, so it mixes with the ambient air behind and you avoid the feeling of draft.
Your windows don't look so old. It must be said that aluminum windows, even those currently coming out have a cold contact due to the materials: aluminum.
By their design: sliding seal on the rail, the seal is less effective than conventional openers, which is made of PVC which is not a cold material. But hey, it's more practical and more aesthetic. And there is not much to gain.
You should know that the heat rushes to the cooler parts where the utility of the curtains to thwart this exchange. This is what I did at home. My bay windows which are only 4 years old are very similar to yours, and despite this: thermal break, contact with the aluminum is cold.
It's like if you have a tiled floor, when you walk barefoot, you're cold, on the other hand if on the floor, there is carpet next to it, the latter is at the same temperature, and you don't feel cold: it comes from the nature of the materials.
To improve the sealing of sliding glass doors, you can take inspiration from this video. I did it at home and it's quite effective.
Attention, to make a notch for the flow of water, and make it a little more important on the side where the wind throws the rain (in the east in principle).
You can also put the joint vertically on each side. It is very easy to do and very inexpensive.
Your floor is tiled, many people put floating floors to cut the cold and to have more comfort at least in one part of the house. As it is not cold, it does not capture the calories from heating. especially with electric heating. others put PVC coating that we simply put with a little double sided on the edges, there too it cuts the cold.
Perhaps you could put an aesthetic curtain against the open staircase for the winter to limit the flow of heat. Take a test by joining well at the top for at least two days to see what it looks like.
Otherwise if you are going to see an electrician for a double flow, tell him that you want something correct, but not too expensive, that you are limited and that you may be forced to resell in a few years (even if it doesn’t is not true), so you want something not too ruinous. So it could slow the addition. Otherwise it risks being full pot.
That's a whole bunch of suggestions