The NSX-T API Guide is very light on the topic of available widget types and their configuration.
Hey drfooser,
By custom dashboards you mean created by you? Or to create custom ones like using vROPS?
Here you can see a post of creating NSX-T dashboards by a user but is not like a design guide: https://mankowit.se/2019/11/nsx-t-2-5-custom-monitoring-dashboard/
I am afraid I could not find anything.