There are two possibilities that come to mind:
Libinput or the Linux kernel.
Since you had tried installing Xorg-Synaptics trying to troublehoot the issue, it seems unlikely to be Libinput.
You might try either an earlier kernel or a later kernel... Given that at this point, you should be using the 5.13 kernel on Zorin OS through the updates, and it being a 2015 model, I would not assume that a later kernel would help.
Try the 5.8 LTS kernel:
sudo apt install linux-headers-5.8.0-63-generic linux-modules-5.8.0-63-generic linux-modules-extra-5.8.0-63-generic linux-image-5.8.0-63-generic
Reboot and use
Advanced Options for Zorin to boot from the 5.8 kernel. Once you reach the desktop, double check with
It should be 5.8.0-63
Then test your touchpad scroll.