If you find yourself programming an eInk display and a microchip in order to improve your procrastination, it is time to stop working on the project, get a physical timer, and work on the thing.
If you feel inclined to shop around for a timer before getting to work, I'll save you the search. These work great. https://www.amazon.com/dp/B07TLC9SFZ (but any timer will do).
Go do the thing. You're worth it.
I tend to like quiet visual timers, though.
Something like:
https://i5.walmartimages.com/asr/63f18bde-f179-4b8e-a32a-8e4...
OP is quite clear in the writeup that this is a project for the sake of trying new tools to make something for a friend.
A fun hack need not be constrained by meeting a real market need. It can just be fun.
Pomodoro timers are designed to increase productivity. But once of the things that many people who try to increase their productivity discover is that they sink so much time into these tools that they are less productive. Because the underlying procrastination isn't solved, and these tools are a wonderful outlet for procrastination because it feels productive.
That may indeed not be OP's problem, but anyone looking at this saying "I need to be more productive, I should make one of these" would be better off with the tomato.
Personally, I like the look of these disc-based ones: https://m.media-amazon.com/images/I/618WCXnyfXL._SL1500_.jpg
At least, they matter to me. I “smartened” an energy meter with an ESP32 and a photoresistor that measures every blink from the energy meter. It’s really crude but it works and everytime I go in the garage, I see the little device blink and it brings me joy. It brings me determination to pursue the next project and motivates me to no end. I know I can build stuff and that I can succeed. I know I can progress in life.
It will probably sound stupid but that’s how I interpret the author’s project.
If it was overengineered and trivially redundant, it was relatable and tactile.
I'm gonna go on a whim and say the habit doesn't last cause you cannot truly depend on them. My watch never leaves my wrist, it never fails me, it is just a "dumb" one.
[0] https://www.amazon.com/product-reviews/B07H59ZL1L/ref=acr_se...