Similar story with me. I started with Pomodoro and I can live with it but wanted something more flexible. I relied on the idea of timers, but without a hard break or a loud noise demanding me to break my flow.

In fact, a physical hourglass timer helped me build up a habit. Now, I don't even use it and have tuned to taking regular breaks without fixed 'Pomodoros'.

I wrote something in that line of thought at https://brajeshwar.com/2023/timer/