Welcome.
Try a two step custom filter.
Filter 1
Ancestral filters >> Ancestors of <person>
Filter 2
Descendant filters >> Descendant Family members of <filter> match
When you create the Relationship Graph, select Filter 2 for the Filter option.
I do recommend that you upgrade to Gramps 6.0.3. There is a known filtering bug in 6.0.1 fixed in 6.0.3
You may be interested in the addons Graph View and the Family Tree View. The FTV is available here
PS: 400+ people is a small tree. It does have the advantage that reports can often handle the entire database.