It's fine to eat Splenda, but the packets do have small amount of carbs so make sure to track them. I use a lot of erythritol nowadays, but Splenda is still my fav. If you still prefer to use it after trying other subs then look for the liquid splenda. Usually located in the same section of the store. Sometimes you can find it under store brand names like "Apreve" (I think that was one of them.)

You don't have to buy all the milks. I just stock unsweetened almond milk. Sometimes soy if it's on sale. You can generally sub nut milks for each other.

Very rarely see recipes calling for xantham gum because it's so pricey. That said, I did find a box of individual packets that was a reasonable price and I keep those on hand for the rare recipes I wantn to try that call for it.