Are you all using the incremental static regeneration API to accomplish this?
Yep. It's great, if hackish. A good overview (https://www.smashingmagazine.com/2021/04/incremental-static-...) or Vercel's own docs (https://vercel.com/docs/concepts/next.js/incremental-static-...)
Vercel aside, I think Cloudflare by itself works similarly if you configure it to "cache everything", including HTML pages, on a revalidation period. I find this a great balance between serverside rendering and static builds. Specifically it allows you to rebuild certain pages as things are created/updated (new or edit blog posts, products, etc.) without having to trigger a full rebuild every single time.