I love Raspberry Pi projects, and I have not worked on one for a while. I don't have anything to do right now, so I may do this anyway. I am experienced with Python so I am easily able to follow the guide you posted. I have created interfaces in Tkinter before, so I know how to do that.
In your description, you mention a web interface. Would that be to control the timer remotely, or to control the timer on screen? Both things I can do. For the former I would just run a Flask server with buttons that did things. If the latter, I could either write the whole thing in HTML and JavaScript, or use Tkinter to create an interface in Python.
I'm available to start immediately. Contact me and I'll make a start.