Hi Hannah! Wow I love the kitchen! I wish I had seen this earlier so I could suggest but I love the grey tile, it looks like a successful choice! You could add a marble cheese board to the island. The grey streaky tones In the board will probably tie in everything else. You could then put a terrarium on top of the marble board!

I agree! Go for what you love. Down the track at resale time, if you feel it's too dated then, there are many splash back materials that can be applied right over top of tile. Is it just the area behind the cook top that will be tiled? I think anything will feel like "too much" at first if you've become accustomed to how it currently is. It's like when you paint a room. It can feel overwhelming at first until you begin to put furniture and decor back in place. A simple way to trial colours is to tack up some wrapping paper, fabric, contact paper (backing left on) or whatever you find that has a colour you think could work. Try use things with different sheen/gloss. Even paint a board and prop it there. Leave for a few hours and see what does and doesn't work for you.

This is going to come down to what your space is like, light, how you use the kitchen, how does it relate to other areas of the house ie. is it open plan, so needs to complement other areas such as dining & family? Here are a few examples of backsplashes with concrete benchtops. Depends on your personal style & taste as to what appeals to you. My 1st reaction was marble, it would give interest & movement but without knowing what else you have going on in the room...

I think a dark grey grout would look great, the wider the gap the more the tiles stand out, if it wouldn't cost too much and you had some spare tiles you could do a mock up on a piece of board , sometimes the only way to really gauge things is to actually see it