No miracles available.
Without logs, there is nothing to suggest.
Arch uses libinput by default.
You used a solution for synaptics.
You have to decide/find which one to use.
Find your current mouse module and maybe add to mkinitcpio.conf (MODULES), rebuilding kernel images afterwards.
journalctl -b -k -g $(lsmod | grep mouse | cut -d\ -f1)
https://wiki.archlinux.org/index.php/Laptop#Touchpad
You may need a udev rule to not let mouse PM(sleep).
Reading is not a sin.
Maybe you get some idea from here.