From an engineering point of view, it is certainly an interesting kitchen timer. If I was making something for my wife to use, I'd be considering a sand glass and would worry that that may be too complicated to use.
Anyway, all I can see is this:
- What bootloader are you using ? Some (especially Nanos) have or had problems with sleep/wakeup.
- The variables you are using in the ISR should be declared as volatile.
- There are some mismatches between the schematic and the code. For example, the pin for the buzzer.