We went down to a 24T granny and then put on an 11-40 cassette off Amazon. That's an easy fix for you. Had to use a Wolf Tooth (has to be the real Wolf Tooth) with an MTB RD as mentioned above, although ours is the old Shimano XTR RD-M952 9 Speed Long Cage Rear Derailleur which came on our Speedster. Ebay if you don't have one. This setup works very well. Though the gaps are larger than we'd like, it was an easy fix.
[Edit]: I hadn't seen the above comments about chain wrap when I wrote this. No, you won't be able to use any cog which any chainring, but (hilariously) you don't need to use every cog with every ring. We set ours up so that the 53/40 won't go, though only one link shy of that, so no damage if you screw up. The small cogs with the 24T are useless, and yep, they sure are. So you just shift a ring without doing the math in your head.
We do use the 53 X 11. Nothing wrong with pedaling downhill, keeps the blood in the legs flowing.
Due to the amount of riding we do and hence chainring wear, and hence scarcity of original 9-speed rings, we changed the left brifter and FD to 10-speed and use the 10-speed 39T triple middle ring and the 10-speed 53T double outer ring. That solved our shifting problems. Only downside is that the 10-speed brifters use very small drums, which results in early cable failure on tandems, so we replace that cable every year before it breaks..