Make a temporary 3cm hole

I rent and could install AC because it is a home improvement. You could ask if it is allowed.

I'm sure if you pay the AC unit yourself nobody will oppose lol

Rent a place with air con installed?

There are the freestanding air conditioners. They have a hose you put in the window. There are plastic seals you can buy that fit in the windows to seal off the exhaust. They aren't perfect but they are better than nothing.

I've just been looking into this in the UK (so Europe, but not EU).

You can now get split units that come pre-filled with a safer refrigerants that a DIYer is allowed to do themselves.

I would still get an electrician to do the electrical side though. 1KW constant load should definitely be on its own circuit.

Yeah that's a good point about affordability. I guess I'd hope for government subsidies or at least supply & demand market forces to address that before it becomes an essential thing, especially if they replace gas boilers too as a move to reduce carbon emissions.