"Should i just use the FSSO agents and be done with it?"
I'd say yes. There are good advantages over polling from FortiGate such as:
1. Offload the task of getting logons to the Collector Agent(s) thus server CPU/Mem
2. More Event IDs are suported polling from FortiGate > 4768, 4769.
From Collector Agent(s) 672, 673, 680, 4768, 4769, 4776, 4624https://kb.fortinet.com/kb/documentLink.do?externalID=FD36424
3. Ignore list > helps with logon overrides done by service accounts
4. Better for troubleshooting
5. Suitable for Large networks
6. Workstation checks for added security
7. Configurable IP address change checks for when user changes networks (typically wire/wifi)
...to name few