Teams + Planner can they replace the Sharepoint project site?

I think that's making really hard work of things. The Project Site template is easy to amend, ensures governance over what you are doing and provides the flexibility of including additional webparts. Remember your project data needs to reside in EITHER Project Online or Planner (not both) and a hybrid option is really the way to go since you can't then get any sensible resource workload data. Planner is super lightweight and has very little functionality at the end of the day. Its suitable only for small groups and small projects, but if you are working in a company environment then Project online is the way to go for sure. 🙂