But before you will start touching php code, I have better idea.
I guess you need Coming Soon page visible only for Guest users, right?
If yes, my concept that I use quite often is:
- Make template Style copy (use Copy button)
- Remove from that template style (1) positions: header, bottom, footer.
- Add that Page (SPPB) that you designed to Coming Soon, as default "home" menu visible for Guest.
- Assign that page (SPPB) from menu settings to Template (2)
- Other menu items you can set to be visible only for Register users to hide them just in case.
Sorry, but my help ends here. I will try to make a video tutorial about that one day.