iMac 2017 + OpenCore nightmare after wrong macOS installer – stuck, installers “damaged”, recovery issues

Hi everyone,
I’m really stuck and hoping someone here can point me in the right direction.

I have an iMac 2017 (iMac18,3).
I upgraded it from Ventura to Sequoia using OpenCore. Because Sequoia was installed on the internal HDD, performance was terrible, so I moved everything to an external SSD. That setup worked fine, except Location Services never worked.

I then decided to do a clean reinstall.

That’s where the nightmare started.

• Internet Recovery would not work at all (no matter what I tried).
• After hundreds of attempts, it finally loaded once and I managed to install Ventura on the external SSD.
• Since I wanted a clean Sequoia install, I downloaded the Sequoia installer on a MacBook — and here’s where I messed up:
  • I did not set the installer for iMac, so it was a MacBook-targeted installer.
• After plugging that SSD back into the iMac:
  • OpenCore got into a weird loop
  • It kept saying OpenCore / macOS was for MacBook, not iMac
  • I rebuilt and reinstalled OpenCore for iMac, but it changed nothing
  • Even completely reinstalling OpenCore didn’t help

After that:

• I tried reinstalling Ventura from Recovery → failed
• Tried Big Sur, Ventura, Sequoia → all failed
• Sequoia always throws: “Installation cannot proceed because the installer is damaged”
• I tried:
  • Checking and fixing date/time
  • NVRAM reset
  • USB install
  • Internet Recovery
  • Multiple USB drives
  • Recreating installers
• First Aid:
  • Shows some errors in Big Sur Recovery
  • Shows no errors in Ventura / Sequoia Recovery

At this point nothing installs anymore.

I really doubt it’s a hardware failure, because everything was working normally just a few hours earlier before this installer mistake.

Questions:

• Can a wrong-model macOS installer + OpenCore corrupt the disk / APFS container this badly?
• Why would Big Sur First Aid see errors but Ventura/Sequoia not?
• Is there any way to fully clean/fix this without replacing hardware?
• What would be the correct recovery path from here?

Any help would be massively appreciated 🙏
Thanks in advance.