Hi Appdev2023,
From looking into this, it does seem the only way to get the relevant selling plan is to get all the Selling Plans, and then search through them in your application to find the Selling Plan with the ID you’re interested in - which could be an inefficient process.
This could be improved if there was a SellingPlan(id: ID) query, as you suggested so I’ll connect with the product teams for this area and submit a feature request for this.
Hope this helps,