It's the floor colour that's the main problem, neither one thing nor the other, and certainly wrong with the benchtop. Leave the wall tiles if you like that sort of thing and find a way of covering the floor probably with laminate planks on an underlay. We have a European Birch golden coloured timber floor but softer grey splashback and it looks fine. It's not an award winner but the floor and cabinet carcases had been there twenty years and it was affordable. Don't even think about painting the floor or putting down mats but a clear plastic runner in front of the sink works well for us.

We had a similar situation with our feature wall bathroom tiles. We chose 600 tiles for floor/walls and feature tile and wanted the grout lines to all line up around the room. However, the feature tile was measuring at 595 and the grout lines looked wrong. I was told that tiles sizes are ‘nominal’ and variations can exist in styles and even batches. We addressed the problem by off-centering the feature tiles, meaning that the horizontal grout lines on the feature wall tiles met half way through the regular tile so it looked deliberate, rather than a mistake. Does that make sense? Would that be a possibility for you? The problem with the tiles being 2mm out is that with every grout line you add another 2mm out so by the end of the row of tiles the discrepancy can be quite obvious.

I’d go with timber or cork. Cork is great in a kitchen. Easy on your feet and joints and things are much less likely to break if you drop them. Especially as you age, tiles cause fatigue if you are standing in the kitchen for too long. Cork is now available in lots of different colours and styles including charcoals and greys that might compliment your slate. It’s not all 80s orange. Of course timber can look great too. Just make sure it’s ok for kitchen use.

Since your tiles are NOT laid on a diagonal, they will not look dated. I would be concerned about the areas of floor that are used constantly. Eg: kitchen. Would the paint, overtime, be worn off. Laminate flooring would be good. If installing vinyl flooring, just be aware that the grout lines will show, if the tiled floor is not prepared before hand. You need to fill or the grout lines, so they are level with tiles. Maybe also installing an underlay before vinyl flooring. Personally, I don't mind the terracotta tiles. With the right wall colour and furnishings they look wonderful.