You could still do all these things before the summer is out. Compiler design is no harder today than 20 years ago. Any text processor you would have created

You could still do all these things before the summer is out. Compiler design is no harder today than 20 years ago. Any text processor you would have created single-handedly 20 years ago is similarly doable. Simple, performant GUIs are easy if you restrict yourself to a single platform. Windows 11 still runs win32 apps, Ubuntu/GTK is also straightforward. For an OS, consider targeting a well-documented microcontroller. Some variants are approaching a similar amount of compute and RAM as 25-year-old PC hardware but have Ethernet ports and/or LCD displays. And if you just want to build a game to say you did, you could build just about anything in the browser using vector graphics primitives reminiscent of DOS.