https://shellbox.dev is a hosted version of something very similar

This sounds great, except for one thing: you can scale your compute (CPU & RAM) as needed but your storage appears to scale with it.

So, if I use a "16 vCPUs, 32GB RAM, 400GB SSD" machine for a period of intense compute, and then want to scale that down to "2 vCPUs, 4GB RAM", most of my storage disappears?

That rather ruins the potential of the advertised scalability.