I've flip-flopped again on the software design, and decided to go for a more hacker-ey approach that uses a tiling window manager + terminal window instead of a browser in kiosk mode. I'm using the following software:

• Xubuntu 18.04
• Xmonad window manager
• XFCE4 terminal
• Vim/Nano text editors
• Feh image browser
• Zathura PDF viewer
• W3M browser
• Ranger/NNN file managers
• HTOP system viewer
• NCDU disk space viewer

I discovered it's pretty easy to hack together a menu-based interface with  Fish shell script, though it's a bit janky. Better options are wiretail (easier, more portable)/dialog (more verbose, more flexible) which allow for decent-looking GUIs in shell scripts.

Off-topic, I also learned about Zenity which is the GTK equivalent. It has Python bindings and seems like a nicer alternative to PyQT (pretty but tedious to code up) and PySimpleGUI (fast to code but hideous as the library tries to look like a 90s WinForms enterprise app).