It's always possible to re-encode video; it's usually called transcoding. However, you lose a bit of quality every time you encode, so you might not gain much in the end. You can offset a bit of the quality loss by encoding at a higher bitrate/quality factor/etc than you otherwise would, but that of course takes up extra space.

You can play in handbrake with AV1 encoding to see how it goes. I think I set the compression to 36 or something.