I'm working on something that. It's reactive instead of proactive (it asks what you did instead of what you're planning to do). Just jot something down for now and deal with your todo apps at the end of your day.
It can do Pomodoro sessions, but I'm adding a couple of variant timers (alarms, actually) that are designed to eventually work around your schedule:
if I have to enter something into an app, I will eventually forget to do it. and if I miss entering too many appointments I'll eventually have to declare bankruptcy
not trying to be negative about your idea. but the best app is one that will know automatically what's on my schedule, without me having to enter it explicitly. that way I can't forget. I have no idea how to do this, it doesn't really sound possible. Just trying to convey my experience with projects like you described
You won't have to enter your schedule - there's APIs for Calendars. You'd just have to determine which calendars are considered "busy".
As for upkeep, I found that the app is self-motivating. It's not measuring anything so if you miss an hour or a month it doesn't matter, there's no overwhelming guilt. The only thing you miss is being able to review what you worked on.
It can do Pomodoro sessions, but I'm adding a couple of variant timers (alarms, actually) that are designed to eventually work around your schedule:
https://radleymarx.com/2021/time-journal-wireframes-round-tw...