SafeSearch does not block Ecosia
The SafeSearch option does not enfore strict search on Ecosia, nor does it block Ecosia altogether. I think Ecosia does not support enforcing strict search on the DNS level yet, but I think Ecosia should be blocked if the SafeSearch option in NextDNS is selected.
I have no expertise but I know that others like cleanbrowsing.org are able to force it on Ecosia. Apart from this I prefer nextdns because I can block individual websites for free. It's as if they simply forgot to include Ecosia. On a reddit thread I have been warned that search.brave.com is not forced either, and the suggestion is that if an engine can't be forced, it should be blocked full stop