Kali installed on USB, but grub is missing

Let me start off by acknowledging that i might be slightly out of my league using Kali, however im spending multiple hours per day educating my self more and more. Coming here for help is kindof my final move, i've tried it all and searched high and low without a solution. My issue is as followed:

I used to have a full install of kali 2019 on my external SSD, which worked perfectly. Turn macbook on, boots into macOS. Reboot macbook and holding option-key, i select the UEFI which shows up, my Kali system boots from the external drive and asks for my crypt key, enter the pass and everything works as it should.

However... i lost the SSD and have been out of kali for some time, so im starting over with a fresh install of the latest version on a fresh usb. Did everything as i did before, without any issues or errors during the installation process. I boot my macbook with option-key, load up the UEFI partition of my kali-live usb, start the graphic install, guided partitioning with full encryption onto my external usb, and then wait for an hour or two for everything to be finished.

After the installation my mac reboots, however it boots straight into grub and then my kali drive. If i unplug the drive, it again boots to grub and stays there. That part isnt an issue, since a simple reboot while using the option-key will let me choose the internal drive and boot into MacOS without an issue.

My problem is, that even after re-doing the entire installation process multiple times, even doing all the partitioning manually, for some reason the kali installer keeps throwing the grub onto my internal drive while i specify that its destination should be the external usb.

Is there a way to fix this during the installation itself which i might have missed in the latest kali 2021.3 installer, or is my only option to install grub to the kali usb manually by booting from the live usb, and then formatting my macbook again for the third time to get rid of the grub thats installed there?

If this exact issue is already noted in another forum or website, i'd appreciate a push in the right direction.

Are you sure, you have only your two USB sticks. One for the booter and one for to install on? If you have any sort of hard drives in you computer, the system will not install GRUB on your USB stick. Either you can reprogram the booter or you need to remove all storages! Only way.

To clarify, my macbook is the actual hardware (which indeed has an internal SSD which now has gotten the grub install on it, even tho during installation i specifiy to place the grub install onto the usb device.

So essentially i boot my live usb through my macbook, to install it on another usb. But like i said, in kali 2019 this worked like a charm. But i cant get it that way by using the 2021 installer or live iso.

Is there a way to install the grub afterwards on my usb, after its already fully set up with crypt? I just now tried running the expert installation from the kali installer, and only installing the grub. But it still installs on the internal macbook SSD as opposed to the plugged in usb.

There has to be an easier way to get this done simply by using the installer, i mean if it worked with the 2019 live usb installer, it should also work with the current version since its still the same macbook i use in the process. Removing the internal storage of the macbook is not an option.

(where sdb is your USB drive) should work when booted from the hard drive with USB drive connected. However, since GRUB puts its config files in /boot/grub, where those will go depends on what kind of installation you have in the USB drive.

If it is a live USB, the /boot/grub folder includes a loopback.cfg and installing the rest of grub there may have unintended effects. You can mount the USB boot partition (assuming sdb1) and specify where the rest of grub goes by