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To be clear on cost, can you please clarify whether all project members will need Premium Planner or just the project manager, i.e. can a project member work on the tasks in Basic Planner while the Project Manager works in Premium Planner?
For basic plans no additional licenses are required beyond the M365 ones - for premium plans then Planner Plan 1 at least would be needed for the PM, but team members could still interact with no premium license - if they weren't acting on the premium features (setting additional field values would be an example).