Have you upgraded the hard drive to an SSD?
You do need to "guide" the Big Sur OCLP install, so it has a chance of completing: watch the install, make sure be there for restarts. there will be 3 or 4 during the install. Make sure that you quickly press the Option key during the restart, so you can make sure that the EFI boot is selected first (press enter), then making sure that the correct install is then selected. (On each restart, the Option-boot choices will usually be slightly different. The important choice is the first one, for the EFI partition, the one that will follow after that is usually correctly setup by the installer - but, in my experience, the installer often will skip over the EFI boot, resulting in an installer fail. You can fix by making sure to manually choose the EFI on each reboot during the install) I have a couple of 2012 minis, that I keep updated with OCLP installs. If I miss one of the restarts, and simply allow the install to do whatever it pre-selects, the install almost always fails.