Not exactly an alias but a short script. First, get git-revise which is a replacement for git rebase, and fzf if for some reason you don’t have it yet. Then make a script in your ~/.local/bin called git-f or whatever you’d like:
Now hack away in a branch, make some commits, and at some point you will realize you want to modify an earlier commit. Use git add -p to add the relevant lines, but then instead of making a fixup commit just type git f and pick the target commit from the list.