> the default (which I've never messed with personally) essentially considers commits in tracked branches from remotes to be "immutable", meaning that anything purely local is "mutable"

The default for what's immutable is actually `main` together with untracked remote branches [1]. I agree that what you suggested should be the default though. You can change it by adding this line to your `jj` config:

  [revset-aliases]
  "immutable_heads()" = "builtin_immutable_heads() | remote_bookmarks()"

(To edit your jj config, run `jj config edit --user`.)

[1] https://jj-vcs.github.io/jj/latest/config/#set-of-immutable-...