From my experience garlic doesn't need to be rotated unless there is a particular problem noticed the previous year. I've even heard of farmers growing alliums in the same field for 30 years straight without issue and some even report better yields in established fields.
If you want to rotate a classic three year garlic crop rotation is tomato family, broccoli family and then onion family. Garlic is a light feeder so you can rotate it after any heavy feeders without issue. I've read mixed reports of legumes with garlic and onions. It seems never plant them together as companion plants and don't plant legumes before garlic. However, I have read a few times that legumes after garlic with another crop in between can be beneficial. Something like year 1: lettuce, year 2: fruits(tomato/squashes) year 3: roots (carrots, onions, garlic), year 4: legumes (beans/peas).
I don't have any experience growing hemp, so I can't say how it will do in a rotation with garlic but I don't see any reason the two wouldn't grow well in rotation as long as you are adding compost to the soil between crops. As mentioned in the comments above I think the most important factor in deciding what is planted next is more dependent your local climate and how much time you have to grow your desired next crop than anything else.
Finally, you mentioned alfalfa, and I think it would make a great cover crop between garlic crops. Some farmers add alfalfa pellets to their garlic beds to add extra nitrogen to the soil.
