I would focus more on the onboarding part. Creating a natural scenario "you want to sign up on this app because a friend suggested it to you - can you please do that"
- how are they clicking through it?
- are they reading the text?
- some steps that are of more interest than others?
- to the user get stuck somewhere?
- how long time does it take?
if you want to try out some tasks thats in onboarding, try to make it as natural as possible, don't make it seem like you are testing out the onboarding steps (i.e taking very specific tasks).
In the end - I would have some interview questions - how they perceived the onboarding, how they would continue to use the product, and if they have any questions further on how to use the product.