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:

#!/bin/bash
REF=${1:-origin/main}  # adjust to your favorite trunk branch name
COMMIT=$(git log --pretty=oneline ${REF}.. \
         | fzf --preview "git show -p --stat {+1}" | cut -d' ' -f1)
if [ -n "$COMMIT" ]; then
    exec git revise "$COMMIT"
else
    exit 1
fi

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.