jj wouldn't have any issues with supporting this workflow, if you could replace magit's git commands with some jj ones it'd probably just work. IOW the only thing that's missing is elbow grease.
actually, with jj's seamless rebases, it'd work exactly as you'd want it to (but maybe not how you'd expect if you're used to the git way) when aimed at a commit in the middle of a branch: pick lines which stay below, above or in the middle commit, which you can edit transparently.
The thing is that magit have a lot of commands that makes rebasing a breeze. Pressing c, then s, will allow you to select interactively (from the commits log) a commit to target when doing autosquash. That will create a new commit with the relevant message. Pressing c, the S (shift+s), will rebase immediately, squashing the current staging area into the selected commit.
Interactive rebasing is a breeze. Editing commits is very fast (reordering, dropping rewording, splitting,…). Same with dealing with merge conflicts (Emacs have various merge packages builtin).
The same speed Vim brings to editing, that’s nearly the same speed you get with magit for git.
actually, with jj's seamless rebases, it'd work exactly as you'd want it to (but maybe not how you'd expect if you're used to the git way) when aimed at a commit in the middle of a branch: pick lines which stay below, above or in the middle commit, which you can edit transparently.