FortiGuard DNS no longer supports ANY filtration (Malware/Spam/Ads/Phish) for free. The DNS without a licensed Fortigate is just a normal DNS, with a licensed forgate on the gateway IP it activates not only the above, but also SNI based Web Filtration.
So I do not recommend FortiGuard DNS as a free option unless you are just looking for a secured, corporate focused DNS with high reliability.
Pi-Hole is by far the best option for everyone anyway.