I would probably use the bathroom. I also have a tile with grout floor, and it's slightly uneven, but not enough to throw off my scale in any serious way. This might depend on the type of scale you have. I think if you weigh yourself in the same place on the floor every day, it will be consistent enough over time.
If the sun room is truly the best floor, you could weigh yourself in the same clothes every day, and then weigh your clothes (dressed in something else) to learn how much you weigh without clothes.
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