Professor Oak's first name is Samuel.
From what I have found, Professor Elm's first name is currently unknown.
From what I have found, Professor Birch's first name is also currently unknown.
Professor Rowan's first name is also currently unknown.
As you said, Professor Juniper's first name is Aurea.
As you also said, Professor Sycamore's first name is Augustine.
Source (Oak)
Source (Juniper)
Source (Sycamore)
If you would like me to add other professors, such as Professor Ivy, please tell me.