In all fairness there's a huge chasm between the OPs property and a grey sterile home
I agree. I like the lounge, but it just has too much "stuff".
Mine looks quite minimal compared to yours, but I still have pictures on the walls, photos, books, DVDs, CDs and plants, just not nearly as many. And everything has a home. Nothing is just stacked on the floor.
I would suggest that you keep it looking like a home rather than a sterile flat, but put anything that can't go in a cupboard or on a shelf into storage.
I would concentrate on the kitchen first. The work surface on the right is unusable because it is so full of clutter. I would remove all the cards from the cupboard doors because it just makes the place look claustrophobic and untidy, and put the books in the kitchen into storage and put the dishes on the worksurface on the shelves. Then I would paint the wall.
Your issue is that you have too many books and pictures for the space you currently have.
And what was the estate agent thinking of when they took a photo of an outside area full of rubbish?
And, sorry, but the bathroom just hurts my eyes.
I'm glad that your DS has completed his treatment, and I hope he stays well. Good luck with your decluttering and eventual sale.