To change to cavity doors you will need to remove the wall each side of the doors to accommodate the cavity housing. Normal sliding doors would make a difference but the problem with them is they never stay totally closed and they do become quite noisy and annoying. This problem also applies to double cavity doors. The other option is to use bi-fold doors, they won't intrude into the room as much. Do you really need two doors there at all and is the glass above the doorway necessary? My personal preference would be to fill it all in and just have the one door to the room. You could even re-use one of your doors.

FormFunctionNT I love your lego door! So innovative. This idea may be a little busy for the other door, but just putting it out there... You fill it with squares of the 4 colours, but randomly leave 4 blank. Almost like 4 have left and moved 'next door' if you'll pardon the pun! Sorry, I couldn't help myself.

Don’t over think it. It has to work in the room and on that facade. Windows should be same height at top and same frame If you are really worried change the 2 panel to 3.

Many many modern builds complain of upstairs being too hot and downstairs being too cold. I think a door is a good idea but I’d couple it with rethinking having one heating/cooling system for the whole house. Speak to an air con professional. Ducted downstairs and splits upstairs maybe more suitable.