Honestly for goodness sake
you chose to put 6K into a property you knew you would not be staying in forever. You could have lived with the tatty carpet and the magnolia walls (which was put on top of the plaster by the way, so cheap or not it was decorated), or you could have gone through reasonable wear and tear channels to have the landlord fix at their cost or repay you.
You have been silly to have put all the work into this property out of your own pocket. I sympathise with the useless LL problem, I've had them but you spent so much!
You agreed to put things back how they were, to not do so would be wrong. If that means you suck up the costs and just paint the walls that's kind of tough because you agreed to it.
If you take everything you've put in out, that would be fine and the LL wouldn't have a leg to stand on provided you replace everything with cheaper alternatives - the things like fittings and curtain rails etc. It'll have to be made good.
You're sounding quite entitled tbh @QuantumDog2 and while your LL doesn't sound like the best in the world, this is an issue of your own making in a way and you shouldn't have paid for normal wear and tear from your own pocket. It doesn't entitle you to throw the agreement out of the window just because you feel hard done by.
Your partner sounds practical, if a little understandably fed up to suggest ripping out the floors etc.
Lesson learned not to spend loads on rentals I guess.