If you run the following in the debug console then close Anki, you may be able to squeeze out a bit more space savings
e=mw.col.db.execute
mw.col.save(trx=False)
e("pragma page_size=4096")
e("pragma journal_mode=delete")
e("vacuum")
mw.col.db.begin()
If you’ve clocked up close to 4 million reviews, I am very impressed - you’re only the second user I’ve come across with such a large amount of studying. I ended up recommending to the other person that they back up their collection (so the old history is not lost), then remove review history from more than a few years back to keep the collection size manageable.