In speech in the US, it’s perfectly natural to drop the ‘at’ in your example. This also works for ‘around’. I start around 4. It doesn’t work for ‘approximately’ though. ‘The second half should start at approximately 3:30. ‘ There’s no harm in including the ‘at’, though.

Hello there! Yes, it’s obligatory as it’s the preposition that goes with time of the day and just “about” is not enough. “About” just means that it’s not 9 exactly, but around this time

The first one

At is optionanal, especially since you also used "about" making it more approximate, if you say "I get up at exactly 9" then you would need to use At/@.