Since you refer to your pages being on the bottom I assume you mean that you’re using Glide Apps (not pages) and that you’re referring to Tabs.
There isn’t a way to have sub tabs…but, you can have as many tabs as you want. You can also place them in the side menu if you feel space is getting cramped.
If you want, you can combine purposes into a single tab and then switch between the purposes using a choice component as a menu.
Here’s an example of one I made that I styled with some CSS to get the effect I want. Note how the content of the tab changes when selecting a new choice:
All this to say, you have options.