I think the solution you end up choosing will depend on the user's optimal outcomes.
Adding a home page selector might be a solution to presenting all calculators without having to use menus, but you probably want to look at how users are working with the calculators on your app. Suppose they open the app to make a quick calculation multiple times, and every time they have to relocate the right calculator and then enter the values, this one extra step adds up after a while and makes the experience less gratifying.
Additionally, if users make multiple calculations during a session, then the homepage solution would mean they must go back, select calculator, perform the operation and go back again to select the next, this will also add up and increase friction.
Simply put, users aren't looking to use the calculator, or even produce the resulting calculation, they are instead setting and getting better at their rowing goals, so the focus must be on the most effective way for them go back to their activity.
Consider swiping and other gestures that allow users to move back and forth between different views. Not only it is preferred for situations where users are busy with other things, but it is easy to memorize where things are after repeated usage. This will also make it easier for them to discover your premium calculators.