Dual boot on single drive

Hello, I've managed to get Big Sur running and all on a partition, except bluetooth, but that's another topic.

Currently, I boot into the macOS partition of my drive through the use of the custom EFI, that I had to make for installing macOS. Therefore, every time I boot into macOS, I need to go to the USB openCore boot loader, and there I can boot into macOS.

Is there a way to let the BIOS see macOS as a booting option, without erasing the whole drive and starting from square one? It currently only sees "windows boot manager" and the SanDisk USB.

Thanks for the input!