15 square meters isn’t the same as 15m×15m, it’d be a little smaller than 4m×4m. Still a big hole.

Have you looked into Ubiquiti equipment? They’re generally pretty solid, and might have everything you need.

Yep, you just have to set Keep as your default note service in the Google Assistant settings.

Shared notes in Keep sync very quickly in my experience as long as you have a reasonable internet connection.