Dear GinTonic88,
Thanks for reaching out to us here in Microsoft Community.
I understand your confusion that you want to select or mark multiple rows of emails at a time in New Outlook, please correct me if I have misunderstood!
I've tested it locally and it selects all emails by simply pressing Ctrl+A. PgDn and PgUp help you jump through emails quickly, but they don't collaborate with Shift.
To see the full list of supported shortcuts or to select the version of the shortcut to use, you can click Settings - General - Accessibility, “click here”

I hope my suggestions are helpful to you.
Feel free to contact me if you have any questions, I am here to help you further if needed.
Best Wishes,
Stan - MSFT | Microsoft Community Support Specialist