You should be able to jj split --interactive change once you have it in a series of many. Afterwards you can move them around and as long as they are not overlapping you should have no conflicts

One thing that I did adopt was having a top-level merge commit common for all parareal stream of work. It has been linked somewhere here in the past weeks

I've read that too however it did not work for me, not at the beginning. I did start small with what I need rather than trying to replicate someone else's workflow. Eventually I landed on hacking hacking hacking with jj new or jj split --interactive to organize things before pushing

It hurts as long as you think in git terms. The moment you forget that it starts to make sense. I see a huge benefit in my day to day for workflows that involve moving a lot of changes up and down the log and I do embrace less friction with signing commits.