WHOA, this makes absolutely no sense. As you know, I tried installing OpenCore many times and could never get the EFI boot screen to show up for the life of me. I knew I'd probably have to inject the GOP, but man, I'm not that versed in those kinds of things.
So I went Jesse's Flying route (install), and holy moly—it was too easy! 😍
I built a Monterey USB installer with OpenCore on the same stick (the usual). With no drives installed, I plugged in the USB installer with OpenCore, powered up, and reset the PRAM after hearing two chimes. I released the keys, and then the EFI boot option showed up (WTH), thers no GOP. I immediately shut down the computer, installed the new SSD in slot 1, and the computer booted right back into the USB installer with Monterey, and not the EFI screen I just saw.
Monterey installed without a hitch. I did my post-install steps, and this is where it gets really weird. After another reboot, OpenCore asked me if I wanted to install the bootloader on the drive so I wouldn't have to keep the USB stick plugged in. Then it prompted me to reboot while holding down the Option key and follow the usual process of assigning it to the drive.
Well, here's where it gets totally WEIRD. BACK TO THE SAME BOAT.
I installed OpenCore to the drive, removed the USB stick, and held down the Option key, but the EFI boot option would never, ever show up so I could assign it to the drive. Even when I put the USB stick back in and tried again, it still wouldn't appear—only the Monterey boot option showed up. I held the Option key down for a long time both ways. (I GAVE UP.) I just left the USB stick in.
I then removed the USB stick, and for some reason it automatically booted without the EFI boot screen. How it installed on the drive ???
So now there's just the two MacOS's that pop up without a EFI boot screen. Yes, I also installed Sequoia, and the same thing, never an EFI boot screen????
ALL GOOD, just two macOS options to choose from.