You might be interested in tangled.sh, a Git forge that recently added support for stacked PRs with jj[1] (or anything that supports the Change-Id header i suppose)!
[1]: https://bsky.app/profile/tangled.sh/post/3lptwcb47kc2u
Nice - I have no choice but to use GitHub at work, but I'm always glad to see competitors.
thanks for the mention! I was just composing a reply haha.