If you want to test other distributions, I strongly recommend that you do so by running them as virtual machines, using your Manjaro as the host. VirtualBox is excellent for this, and the learning curve isn’t too steep. You do need a fairly strong cpu, I recommend at least an I5 or equivalent and at least 8GB of ram so you can split the ram 50-50 between your host and virtual machines. Any time you are installing multiple distributions you are messing about with your partitioning and with grub, which has a bunch of pitfalls that aren’t there with virtual machines. If you mess up your VM, it can be deleted with a couple of mouse clicks, and there is a snapshot option in the VB Manager to reload an older VM if you happen to mess up one that has been working.
Assuming both your Manjaro and your Zorin are installed uefi, you can try installing rEFInd on your working Manjaro and see if it picks up your Zorin boot files. Read the rEFInd manual before you install.