When I look at a FIRST challenge with whatever team I am involved with, I like to have my options open, which is why I am a big fan of a super simple drive type that can be built off of. The T-slots and the extra attachment points seem to make the design you built very modular though, which I like.
Let me get to the point, T-slot is heavy. Switching to a gusseted or welded tube frame will get you to a happy place. If you have sheetmetal options, you can go with that. I also became a fan of the kit frame working with 2702, we built a very solid, and reasonably light drive with it alone (much lighter than what you have quoted). I am sure you can find plenty of pics of the structural tubing design, whether it is a WCD Derivative or something else, they all seem to work pretty well.
I shall provide my take on the tubing frame as well, I have posted this before, but I challenged myself to make a warp resistant (keep welds away from the drive components) gusseted frame that was relatively light. Heavy 254 influence below. This has never been built, but it sums up a lot of peoples ideas.
Probably the big thing I tried to improve with this frame was robot torsional stiffness. By that I mean the frames ability to keep the two drive component rails parallel. The stringers between the drive component rails really help if you don't put a top box on the frame like you have on your design. That is the advantage you have at the expense of weight, nice torsional stiffness.
Look into Team 221 LLC's bearing block tensioning system as well as 254's. They are both good ways of going about things if you choose to go live axel.