ServiceSets may be worth looking into – admittingly I don’t know much about them.
The problem with this isn’t so much the OID as it is the differing SNMP port numbers. We get around this by relying on the Fortigate model as a custom var – if it matches one of a few different model numbers that we support it gets checks applied like:
I found this guide on service sets, surely enough, and merits more looking into.