You can either include grouped rooms or set individual alarms for different rooms set for the same time.

If you use Apple iOS as your controller devices, then you can schedule an alarm (station/track/album/playlist etc; saved to the ‘My Sonos’ tab) to play to any ‘group of rooms’ using the 3rd-party ‘Soro App’ (Note: I’m not affiliated to that App, but I do use it myself for the other things it can do with Sonos products).

Yes this is correct. It’s also something that users have been asking for ever since the alarm feature was added. I doubt it is a technical limitation so my assumption would be that Sonos’ attitude is that the desire to not break existing groups/stop currently playing on other speakers or groups trumps adding this as a feature.

Grouped rooms functionality require to have the rooms grouped at all time.

Setting individual alarms does not sync the music 

Yes that’s correct but Soro accesses the Sonos API and it can take these ‘example’ steps shown below, which are ‘grouped’ into an iOS shortcut ‘action’ which you schedule to play at times/dates you choose:

• Step one - Soro groups the rooms you choose.
• Step two - The queue is cleared on the Group Controller.
• Step three - The volume and any required EQ settings are set.
• Step four - The GC queue is populated with the tracks/playlist/station you choose.
• Step five - The timer is set to play for whatever time you select.

You can even shuffle the queue, so that it starts playing something different at each alarm activation.

These steps happen of course in an instant and the above is just an example. Here is a screenshot showing the above that I created a few seconds ago using the App… it takes less than two minutes to create this type of macro (it’s mostly drag ‘n’ drop for the user).