So the equivalent of "git gc --aggressive" - but done properly - is to
do (overnight) something like

git repack -a -d --depth=250 --window=250

where that depth thing is just about how deep the delta chains can be
(make them longer for old history - it's worth the space overhead), and
the window thing is about how big an object window we want each delta
candidate to scan.

And here, you might well want to add the "-f" flag (which is the "drop all
old deltas", since you now are actually trying to make sure that this one
actually finds good candidates.