I haven’t tried jj yet, but from my reading of the article, 80% of the problem is that jj split is backwards for some (most?) uses. You seem to want to split a change such that the newly split piece goes before the part you leave alone, and jj split wants to do the opposite.

Certainly when I split a change up in git or hg, I’m usually trying to break off a piece that goes before the rest. TortoiseHG’s committer works like that — you select the parts to commit, and the rest stays uncommitted in the working copy.

So maybe the only tooling needed to make jj better is a mode (or maybe a default) that reverses the initial guess as to which parts of the change go where.