My bathroom is tiled right to the floor and I grouted it with normal grout but added an admixture to it to make it flexible and then silicon after it had dried, if the floor is tiled then you should be using flexible grout on that too so you may have some admixture left, its not dear anyway for a small tub.
You could fill it with some kind of flexible filler and then silicon after, either way would be ok
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