DJM 900 SRT with Serato cuts off for random seconds

You may have the 4.0.0 drivers; check that and update to 4.0.1 if you are.

That I forgot to mention on my initial post, but the drivers are for sure the latest. Downloaded drivers from Pioneer website. Also tried after uninstalling drivers to let Serato do its magic but apparently, it installs the very same latest driver for the mixer. Also please note that I'm running fresh install of macOS, so there is no old burden laying on my laptop.

"This same behavior can be reproduced in internal mode" 

Meaning the internal software player of Serato where the audio comes out the speakers and not the DJM?

In Serato internal mode the audio still comes out from DJM 900.

That to me says you have the DJM assigned as the default audio device for the operating system (which it shouldn't be), and that would prevent Serato from detecting it and unlocking the app. Change the audio preference to the built-in audio device.

Sorry I don't quite follow, why the DJM should not be assigned as default audio device? Anyways, I did try to switch the default audio device as suggested but there's no change.

Because if it's assigned as default for the OS, it can't be exclusive to Serato.

If your'e still having troubles with it, you can test with another computer to determine if the fault is the mixer or the computer. If it's the mixer, contact an authorized service centre in your region to have your hardware inspected and/or repaired.

Turns out I did not have the latest firmware after all in the mixed. After upgrade, everything works as expected. Thank you for the help!