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.