I don't think any non-1x style rear derailleur supports more than 42t cassettes. For total range with a derailleur you still can't beat a triple... Look instead to the front where you can install a much smaller inner ring (like a 22t)
I'm sorry your current bike has trouble with racks. It seems like it shouldn't. A trailer hitch is simple, it just goes on the skewer right under the quick release lever
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Genesis 49:16-17