You’ve only had the mattress one night - I’d strongly discourage you from making an assessment after only one night of sleep upon the product. All mattresses will soften slightly with use, even latex and the covering and the non-woven fabric wrapping the springs, although the amount will vary based upon materials being used. Your body will also go through a period of retrogression where you’ll adjust to the new product as well.
If you’re bent upon softening up the mattress after one night of use, you can always make the exchange of the comfort layer. As you don’t seem to be unsure of what “medium” means in the mattress you purchased (“Apparently this means that the latex topper inside is of medium firmness.”), please make sure to make sure of exactly what differences there will be in any layers you’re exchanging should you decide to go that route.
Don’t pay attention to other people’s comments of what they personally think is “too soft” in a certain mattress layer - they have no bearing upon what might work for you. I’d certainly consider the advice from the manufacturer in a good phone conversation. Being a prone sleeper part of the time, too plush of a surface can exacerbate your natural lordotic curve in the low back and cause discomfort, which is probably the reason for the comfort suggestion they made.
You don’t tell what size mattress you have, but a latex topper around $300 doesn’t sound unreasonable for a queen size (if that’s what you have). No one can pick out a topper for you or what specific material you may like, and realize that this topper too will soften with use. There’s a good topper reference post here you may want to read that gives some excellent topper shopping advice. However, adding another variable to a new product that you’re still adjusting to can further complicate your comfort selection process. But it does give you the options of adjustability and ease of removal to create different comforts. You have to be careful about creating too deep of a cradle of comfort on top and sinking in too deeply, especially when prone sleeping.
My two cents and best advice is for you to sleep on the new mattress for at least two weeks and allow the product to “settle in” a bit, and then make up your mind. This forum, and others, are populated with people whose initial impressions are quite different from their thoughts after a couple of weeks sleeping upon a new mattress, and they’re surprised how a mattress they initially thought was too firm actually ended up being quite comfortable. You should at least give yourself that chance and let your body lose its learned alignment from your old mattress.