I truly admire your dedication. To translate an entire book sentence by sentence, just for the pleasure of fully understanding it and having it on your bookshelf, is a whole other level of love for reading. I think it's fantastic that you're using tools to help you; ultimately, what matters is unlocking that story that captivates you so much.
From what you've said about the summary, the plot sounds incredibly intriguing, like a dense blend of alternate history, biblical legend, and adventure. That whole journey from Egypt to Ireland sounds fascinating; I completely understand why you feel like you're on a treasure hunt.
RE: The Book of Tephi/ Das Buch Thepi