That is, nowadays I usually do the same, but through Git Fork [1] (via its custom command system), and it works even better than the terminal interface for me -- which doesn't happen often. But for fixups the "find the target commit in the history and use a context menu entry to flag your commit flow" really works rather nicely.
Also, Fork's interactive rebasing dialog does `--autosquash` by default.
[1]: https://git-fork.com