You can absolutely do that. If you want to upgrade to a paid plan to get some of the features you want, then you can switch at any time through the My Products option in your account or with the help of a GoDaddy Guide. Once you upgrade your site will have all the same customizations and work that you've already put in place, just with the added features. See Website Builder plans and pricing for more info.
I’ve made some use of Kirby CMS. It’s a really well put together flat file CMS. It takes some coding out of the box to get it set up as desired, but then it’s a pleasure to use. Advantages of not having a database include simpler setup, and the ease of version control of the whole site. Statamic is a similar option, though I’ve not spent any significant time using it.
In a nutshell, when it comes to WordPress it is an all in one platform which helps in creating a website, blogs and manage content. It has its advantages and disadvantages. When it comes to specific projects or sites which are to be created it may have pluses and minuses. It depends on the project, user and his requirements and strategies which define which application is to be used. There are many WordPress alternatives with different features.
While there are modules and themes that extend the features and looks, novices find it a bit difficult to create a detailed site with it. However, the complexity of Drupal is appropriate for handling huge amounts of data. This makes it ideal for larger corporate and enterprise websites. Just like in WordPress and Joomla, Drupal also allows various extensions in the form of modules. But the difference here is, unlike WordPress users need to leave the system and manually search for the modules before installing them.