I don't have ADHD but I would find that useful (I'm not a mac user so your implementation is not for me).

I have used pomodoro timers that have a sound (for instance a clock ticking) and that's a useful way to be reminded. Unfortunately the ones I've seen to date have a reasonably limited range of sound configuration options, generally a one second tick. I would prefer to hear a quiet sound every thirty seconds, I'd also be interested in the option of a voice announcing, say, every five minutes elapsed.

The OG pomodoro method uses a ticking physical timer; object permanence and visual and auditory cues are built-in.

As far as apps go, I think your idea makes sense. But if you're working on solving a personal difficulty, I suggest going back to the basics and keeping it simple. The point is to work on what you ought.

Link? Would love this built into meetingbar which I already use to (almost) always not miss meetings.

I thought about building a similar app for myself - for one I want some kind of pomodoro style color shift on a regular clock to help break time blindness.

Do you have a repo of what you built?