Yes thats true, once you start adding interior lighting, PT will become more important.
You can run TM on the latest iMacs however the iMacs are not that great for working in 3d. I would recommend you to look into getting a Mac mini and buying a monitor. https://www.apple.com/shop/buy-mac/mac-mini
The highest configuration of the Mac mini has 10 CPU units and 16 GPU. The amount of GPU units will is the one factor that TM cares about the most. I currently work on a Macbook Pro that has the same amount of CPU and GPU with the above mentioned Mini, and I can run most projects in an acceptable speed. You can take a look at this video to see how the performance is: https://capture.dropbox.com/N7xA6vXEBc0Xewbo
**edit ** for anyone wondering the performance on M2 Max & TM 2023.1.2: https://capture.dropbox.com/WQHc017HWI5RJxEv