That's the purpose of Blitzit (https://www.blitzit.app/), which makes it very well !

Only one task at time with a countdown timer, and a notification sound every 10 minutes !

I quit blitzit because of there UX and they have no integrations, which is really great in Ellie

PLEASEEEEEEE sunsama does a focus mode that actually helps me with subtasks but I like this app better so this would make my LIFE

This would be AMAZING

Yes please, especially in the mobile app! I would love to put my phone in focus mode and work through my tasks and subtasks. Would be even better if it gave the task for the timebox you are in first