Supermetrics is definitely not needed. Your GSC API can access all of the properties you have access to. It pulls from a single property (domain or url prefix). With this you can create a script that pulls the data you need for each individual property and then a script to visualize said data. From your question, it seems you might be newer to GSC API (and likely coding in general) though, so this task is much easier said than done. That being said, Looker Studio could be a much better fit for you. Though.. it will still be quite a bit of work mapping data sources, etc. It's a good idea, but in my opinion, I wouldn't say the juice is worth the squeeze.