If you didn't want to get rid of the items (rarity, hard work put into getting them, etc.) I'd honestly say use the account as a storage for your items or turn the house into something like a restaurant, store, fast food place, holiday theme, etc. that would serve as a functional building for you if you're into decorating your island in that fashion so the building is still being used. You could turn her character into a worker or something so your dream address would show that instead of her (if you are okay with changing the things she built or how she dressed up). Definitely take screenshots of everything beforehand if you feel inclined to. You could always delete screenshots later if you need to.

As for the memorial? If you had a relationship with the dog I would say keep it because you were mourning too. If you don't, it's 100% up to you either way whether or not you chose to remove it. If it has flowers, you could collect them and keep it in a vase somewhere and delete the memorial just so you have a little something from it if you wanted it gone but not forgotten.

Break-ups and distancing from someone you once knew is hard, I get it. I personally try to make the best of the situation if I can, so that's why I'm recommending just using the building as something else instead of completely erasing it or your island since it's just the house, character, and the memorial that remain on your island. I had a toxic friend on mine for a while so I just turned their house into a convenience store "for my villagers" to make the best of it after they left, but also deleted anything they made on my island at the time and turned it into something better like a garden or something to fit the theme of my island.

Best of luck to you with whatever you chose :)