I had the same problem with my Mac mini m2. My MacBook Air m2, iPhone 15 pro and iPad Air 5 already had Apple Intelligence working quickly after joining the waitlist.
I have tried the following:
Check if all the necessary settings are correct, Mac and Siri Language set to US English and Region set to United State.(After you've enabled you can change the region again to your own country.)
I made a Time Machine backup of the current state of my Mac mini.
After that, I completely uninstalled macOS.
Installed the version of macOS that came with my Mac mini out of the box.(through recovery mode)
Set it up as a new Mac.
Install macOS 15.1(If not already installed)
After this enabling apple intelligence was easy. I could pretty much instantly enable Apple Intelligence.
After that, I erased the Mac mini(Reset and start initial setup).
Restored it using the Time Machine backup I made.
After restoring it took about 30mins and now I can use Apple Intelligence.
It is quite a rigorous process, but for me it fixed the issue of being stuck on preparing. I don't know if this will work for iPad or iPhone, but if you really want Apple Intelligence to work, you can try this method though. Keep in mind that it might take a few hours, as you have to uninstall/reinstall/restore macOS/iOS/iPadOS multiple times. It took me about 4-5 hours for the complete process.