Hum, have a M1, M2 and M3 machines all previously running Sonoma 14.6.1
Downloaded the Full Installer of Sequoia 14.54 GB
Make a copy to USB
Launched the Installer on the Original Machine ( M1 ) Yesterday Monday
20 Minutes later - Presto macOS 15 Up and Running
Moved the USB Installer to M2 and M3 machines today Tuesday
Presto, both machines up and running macOS
The Only problem I have is which of the 3 machines to pay with today
To be constructive >>
Start now and open an Apple Support Ticket as they are PAID Apple Employees to deal will these types of issues .
Product Feedback - and make it known to Apple regarding this ongoing issue
Have the computer evaluated by the Professionals who have the Special Hardware Software and Expertise to diagnose the issue and offer possible solutions
Make an Appointment at the Apple Genius Bar or call 1 800 MyApple and have them evaluate the computer
Other location Call Centres
Contact Apple for support and service