Have you tried the THea tutorials on the website:
Online documentation for the Thea Render integrations
www.thearender.com
There's also the official youtube channel. Some of the videos are 10 years old but the principles of the software haven't changed a lot mainly just the interface
Also another youtube archive - again very old but some of it is still relevant:
I noticed this too but i don't anything about it:
Complete Guide to Creating Photo-Realistic Renderings, Mastering Thea Render and Using it to Create Better Renderings.
www.udemy.com