Looking for a guest bed that I can switch from extra firm to a medium-ish with a topper.

I am looking to get a guest bed mattress that is extremely firm, but good to pair with an existing latex topper I already have so convert it to a more suitable average person guest bed.

My parents will visit a number of times throughout the year and require an extremely firm mattress, so I am purchasing one mostly with them in mind (quite frankly, whatever you think is firm is likely not firm enough for them). The ones that come to mind are the Plank and the Engineered Sleep Classic in extra firm. I am looking for something that’s not too thick so it isn’t too tall paired with a 3” topper.

Does anyone know if either of these are traditionally a little more comfortable with the topper? Or if either are significantly firmer on their own for my parents?