Just a (successful) data point: I created a setup with OpenCore and a variety of MacOS versions, which works well. However, I'm doing kext development, and had some conflict issues with OpenCore, so I wanted to be able to boot some MacOS versions natively for testing. Enter rEFIndPlus - all I did was create EFI/rEFInd, copy in rEFIndPlus.efi (release version), the icons, and the sample config file (other than timeout=0, I took all the defaults), blessed rEFIndPlus.efi, et voilà - I can boot any MacOS I have installed. rEFIndPlus always gets control first, so I can boot Mojave and earlier MacOS natively, or I can choose the OC bootloader, which gets me the OC boot-picker, from which I can boot any MacOS (including Catalina and Big Sur). At present, I have no need for Windows or Linux on my Macs, so it's been smooth sailing. I may eventually invest some time to create specific menu entries and hide some partitions (like Recovery and Preboot), but even without customization, it's 100% functional.
I'm having a problem with rEFIndPlus hanging if there are two keyboards attached, but I'm still trying to determine if that's just a problem with my setup (as opposed to a rEFIndPlus bug that's worth reporting). I'll let you know if I think it warrants your attention.
Thanks for creating rEFIndPlus. I also used it as a drop-in replacement for the old rEFInd installation on my MP3,1, and it's plug-and-play, just as advertised.