You can’t make the Senate proportional, or substantially change its make-up, even by constitutional amendment. It’s the one thing that the constitutional specifically says cannot be changed, except essentially by unanimous consent.
Probably the most realistic way to truly reform the Senate is just to gradually erode or remove its powers, kind of like the UK did with the House of Lords.
I always thought a great improvement to the current system is an amendment that allows the House to override Senate decisions with a 55% vote in favor.