My Ubuntu Touch phone (BQ 4.5) is running out of storage. I'm looking for a GUI way to make space, most likely by clearing a .cache folder. I'm happy to install apps, use the file manager, or use any other non-terminal options as a normal phone user would.
Sorry, but there is no app that does exactly what you want; the file manager you mentioned comes closest. Then you can access most files using a GUI and can delete anything you want.
If you want to do more, you need to put your phone in developer mode and run the file manager as root as well.
Warning: you can delete everything when using the root mode, including system files which can then brick your phone. (No, I haven't tested this!)