Good to have a dialogue with you.
I haven't known that Trinity is related to the Egyptians but I can follow easily your assumption it is even older. Why not? Many of our knowledge and beliefs can be followed far in history of mankind. The different interpretations resonate with us - with one more, with another one less.
What I always need is to integrate the insights into my daily life - and therefor into what "matters" with me. Without the connection to my physical existence I find it difficult to take in what I have learned or realized. I want to use wisdom - or whatever you may call it.
In almost every case I come to the point that mythology, psychology, sociology etc. for me only makes sense to dig into when what is stated can be connected to ethics. Ethics is an important Noun for me but even more significant is how I live them out in my relations to humans and all other living beings. Not easy.
May I ask for what and how you use your knowledge and insights? I really would like to know.
P.S. haven't read the book you mentioned.
An example of the Egyptian trinity