How do I actually use Unreal Insights to efficiently find blueprints and functions that are slowing down my game thread?

Did you try the Session Frontend method? I’ve found absolutely no use for Unreal Insights till now.

For that command you were looking for, I used these commands:
-cpuprofilertrace -loadtimetrace -trace=default,memory,metadata,assetmetadata

They go in Editor Preferences > Level Editor - Play > Additional Launch Parameters textbox.