It is exactly for this sort of problematic situation that I have my own personal policy, which goes like this:
One hard drive, one OS. Never two OS’ on the same hard drive (and I’m not talking about virtual machines here). Each OS gets to own that whole hard drive, including the Master Boot Record, or GPT equivalent to the MBR. Each OS must leave alone, and ignore the other hard drives, especially when writing GRUB configs (“installing”) to the boot loaders. This still allows a situation where one can boot from multiple OS’, but this keeps them separated, and out of each other’s hair.
I choose which OS from which hard drive to boot from, not from a grub menu, but from the (UEFI) boot menu which my BIOS generates. On my particular laptop, pressing F8 at boot time shows this (UEFI) boot menu.
My laptop has MX Linux installed to the internal M.2 drive, and this is the only OS on that M.2 drive. Then I have an SSD in a USB3 enclosure (type C, which is a “self-latching” connector, making it a bit safer), which has Pop_OS installed on it. MX Linux uses GRUB, but Pop_OS does NOT use GRUB. It uses systemd-boot. Since each OS controls its own separate hard drive, there is no conflict between GRUB and systemd-boot. My BIOS boot menu lets me pick from either hard drive to boot from, and GRUB and systemd-boot never need to know about each other, let alone conflict with each other.
Yes my idea costs a little more money, but so what. I feel that my time is worth it. I want to live a more hassle-free life, and I’m willing to spend a little more to have that.