Hello
Just a suggestion. You should be able to configure FortiGate depending on your model/version to prevent certain logs form being sent. This may help reduce the amount of log/s being ingested.
Not sure what Model/Version you have so here are a couple link for you.
https://docs.fortinet.com/document/fortimail/7.0.0/cli-reference/820362/log-setting-remote
https://docs.fortinet.com/document/fortigate/7.0.2/cli-reference/456620/config-log-syslogd-setting
Hope that helps