(Why) is a toy password manager a too complex summer project?

I'm an high-schooler who likes to program in rust in part of his free time, in the last monts I haven't
written anything but I want to start programming again in the summer. Last year I did an application launcher with GTK-rs which sort of worked, it was really basic but kind of did what I wanted it to do, so I started to feel less motivated to improve it and add more features, also because I didn't manage to get it working as I wanted in my setup (the main idea for the launcher was to keep it always open in the desktop window, which is ok in most Linux DEs, but not in i3wm).

This time I wanted to do something more "finishable", a project that can be considered an interesting/satisfying implementation just by the fact that it works. I'm interested in math (especially Olympics mathematics and geometry in general) so I thought a project related to it could be more stimulating and approachable in the same time.
Looking for ideas I found a few articles suggesting to implement an encryption algorithm, which seems a perfect base for my project, but I hoped to make something more practical. I thought writing a basic password manager (with no expectations to make it really an alternative to the existing ones, but at least usable) could be a cool way to apply an encryption algorithm, but I doubt it's really as straightforward as it seems. What am I missing? Is there a lot of theory I would have to study also behind the application of encryption algorithms beyond the algorithms themselves?
If you have any other projects ideas, every suggestion is well accepted