Room Treatment suggestions and advice

Hi folks,

I'm coming here asking for advice. First of all I understand I need to do some experimenting myself with positioning, toe in etc, which I will due in due time, but regardless of that I'd like to ask for some advice on some room treatment options.
Here's a link to some photos/drawing of my room (explained later)https://imgur.com/a/C6D3IhC

I have recently moved to a new house. I am renting so my options are potentially limited (even though I could drill holes in the walls to hang stuff as long as I cover the holes after the tenancy ends so don't want to discard this option). I am also in the Czech Republic, nearby Prague, so that may limit slighly where can I buy products from, even though I have already found some online shops with audio treatment stuff (mostly MegaAcoustic stuff coming from Poland).

I am planning on setting up my stereo system in a room that is rectangle shaped, quite long (2.6 meters by 8.5 meters roughly). I have attached some pictures that I will be referencing throughout the post. On one end of the room I have a desk where I'll be working (I will work from home even after the pandemic is over). Then I have a kallax separating the room into two spaces, and I was thinking of having my stereo setup on the other end of the room.

I have placed the speakers but at the moment because of the room geometry and the empty walls, the acoustics are terrible. It's very boomy and echoes a lot. So I'd like to ask you people what decent audio treatment options I could try to try and improve things. I am not necessarily looking for a perfect setup, just something that doesn't suck (as it does at the moment) or gives me the best bang for the buck.

I have attached some photos where you can see the room and some floorplan/3d renders. I have added some annotations on them with numbers so I can easily reference them in here. Units in the photo are in centimeters. (also bear with me, this setup is temporary, I will be getting rack for the DAC/Amp and that cardboard on the bottom of the speakers will be gone).

The first 2 things I would definitely like to do are:
- Have a curtain pole and a curtain to cover the back wall (number 3).
- Have a nice thick carpet on the floor (number 4).

Now, when it comes to more things, I'd like to know what are your thoughts on:
- Current positioning of the speakers (distance from the walls) and current listening position? Unfortunately those doors will be difficult to deal with unless I can find some portable panels I can easily put and remove when I do a listening session.
- Adding some bass traps in the corners by the back walls (between 1 and 3, between 1 and 2)
- Should I also add some acoustic panels on the walls 1 and 2? I think there are going to be some early reflections there (I still need to do the mirror technique to find out exactly where those are).
- Finally, what to do with walls 5 and 6? I have some IKEA billy bookcases to put into the room but they are the ones with glassdoor so that's not ideal as I think they'll be quite reflective. Do you think I should be looking into normal bookshelves, or not have those at all and have some more acoustic panels hanging in there? Unfortunately the gym bars I won't be allowed to remove.

Any other comments or suggestions would be greatly appreciated.
Here's a link to the photos:
https://imgur.com/a/C6D3IhC
And here are the kind of panels I was thinking of: https://www.muziker.cz/mega-acoustic-fiberstandard120-black

Thanks for your time!