windows can and will break any multiboot it gets it's hands on, guaranteed just a matter of time

Dual booting with Windows is always a pain, because Windows likes to nuke and replace your boot menu. The safest bet is keep Windows strictly separated: You create a 2nd efi system partition on your second drive with linux, use a boot loader there and then set everything up to start that as default. And then you configure the boot loader to chainload windows from it's own ESP on disk one. This way Windows is oblivious about linux systems it might try to damage. And you can then set the boot loader menu to a default or to default to the last system booted. (2 separate ESPs on on disk might work, but that is not supported by UEFI, so it depends on your hardware's implementation if they are recognized or if it just stops after having found the first...).

I would assume what you did was install the Linux boot loader (efi file) as the default like removable drives do (so grub's efi file installed as esp/EFI/BOOT/BOOT64.efi which is the default for removably drives to take priority; done via grub-install with the --removable flag, some installations might use this by default...)

AFAIK you can just run os-prober in debian, and then you can launch endeavourOS from there.

People normally warn against dual booting because of the headaches it can cause - you went and fucked up with triple boot.

Are they installed on separate drives? Depending on the exact setup, Linux and windows both generally support legacy as a boot method, so you may be able to just BIOS to select a boot drive.

Try rEFInd. It's pretty good about detecting OSes and auto-populating its menu, and it has a pretty easy conf file format otherwise. Installing the refind debian package mayybe will also register it with the system firmware, I don't recall. If not, it can be set up with efibootmgr (which is not the most pleasant program, sadly).