GroupWise may be the better PIM (even if there is no Mac-Client ...) ... and maybe Reload (GWDR) is a perfect tool to enhance availability.
But review the GWDR documentation ... (and as a addon translate it into the languages of the countries you want to sell it ...)
If you try to install / migrate / configure a reload system you jumps from trap to trap ... it is a pain ...
Because GWDR does not check all needed requirements ... some futher action will fail w/o a message
Even if SLES15 is supported you have to install rpm-32-bit, and rsync by hand:
# zypper in rpm-32-bit rsync
both is not written in the documentation.