In your place,
@OldMacPro2 and
@David403, I would try reinstalling everything from scratch, using the USB installation stick created with OCLP, remembering to install OCLP when finished.
Then reboot the Mac and at startup, while holding down the Option key, install EFIBoot from the stick, then continue with the recovery utilities, format the entire disk, and try installing again.
Warning. If you create the USB stick with another Mac, you MUST indicate the Mac on which you want to install Sequoia from the list of all OCLP-compatible Macs.
I wish someone would give you a better suggestion than mine and faster…
The thing is that just this morning, reading the latest article on "The Eclectic Light Company" blog, I saw that the macOS 15.4 installer creates several problems in some cases.
Toward the end, I read that Apple has made changes to the installer and that, although this is known to cause problems for M1 Macs, it could interfere with unsupported Macs as well. And by the way, this is not the first time Apple has caused problems with new System updates....
So, in your place, I would choose to download and install macOS 15.3.2 to be on the safe side.
REFLECTIONS AND CONCLUSIONS
(Obviously from my point of view that I do not want to impose on anyone...)
After all, Sequoia 15.4 only introduces mainly AI changes dedicated only to Apple Silicon models... Another factor that causes burdening to Intel Macs (but also of M-series Macs) also because we all act as lab rats for the benefit of Apple which is far, far behind with AI, with the new Siri, etc. And so, in trying to gain time quickly, it will inevitably cause problems, depending on the Macs on which we will be doing the installations.
I conclude, even for the above reflection, that I am now 99% in my decision to go back to Sonoma. In Sonoma Apple does not have to experiment with anything and the System has proven to be stable and ultimate for Intel Macs and, of course, especially for all of us who use Intel Macs and OCLPs.
Why, then, want to tempt fate for nothing in return but various problems lurking?....