In addition to our main website, we need to build some microsites(on subdomains/new domains) where we want to reuse all the slices available in a given custom type to reduce our development time. We are using react.js.
This is easier if you're using Slice Machine, which is currently available with Next.js. With Slice Machine, you can store your Custom Type and Slice models locally, and push them to Prismic. That way you can version and share your content models in your codebase. Are you using Next.js with React, or React on its own?
Unfortunately, we don't yet have any tool for migrating Slices and Custom Types to Slice Machine. Let me know if there's anything else I can do to help.