I think it's currently non-selfhostable. You can host your own git server (knot) and CI runner (spindle) but not really the UI/API itself, but they're working on changing it. Currently it's a bit centralised
I have a Next.js app too, and it takes about 1m30 to deploy. it really takes longer to build when your app gets larger, and if I'd had to rebuilt the app focusing on build time, I'd probably go with Remix
(note this is with Vercel's [proprietary] build cache, it takes longer when there's no build cache)