There are lots of ways of doing subagents. It mostly depends on your workflow. That's why pi doesn't ship with anything built in. It's pretty simple to write an extension to do that.
The simple approach is great, chef's kiss, don't change a thing. Orchestration at the harness level tends not to be great anyhow, it's not built for the type of review that's needed.
Or you use any of the packages people provide, like this one: https://github.com/nicobailon/pi-subagents