Hello,
I'm a big fan of white (Dutch) clover for orchards. It's tough, covers the soil to prevent weeds and provides nutrition. A side note is that I often seed very "heavy", meaning far more than the recommended amount of seed. I usually water pretty heavy to keep it going in the summer.
Of course every cover crop has pros and cons. I'd ask you to think carefully about alfalfa because it has deep roots and it's a perennial. By deep I mean several feet deep depending on conditions. I'm a big fan of alfalfa due to its toughness and many other qualities and I've grown it commercially (a long time ago) but as a deep rooted perennial (think 20+ years often in the right setting) you need to be committed to it.
I love legumes for cover crops. I'm also a fan of hairy vetch. It produces a lot of mass. It's great to mow that mass and have all of that green chop to feed the soil and provide mulch. Here's a lot of good info on it:
https://www.sare.org/publications/managing-cover-crops-profitably/legume-cover-crops/hairy-vetch/
If you're planning on mowing it for nutrients for the soil be sure to mow before it flowers because a great deal of the nutrients that would have been in the cut leaves go into flower production and you'll lose those nutrients. One thing that I do is plant a variety of cover crops in the alley ways (each alley having a different one) and see which perform well in my soil. Then I start narrowing the list down to see what performs best for my purposes.
I believe comfrey is a must in an orchard and planting it between trees is beneficial for the mining of minerals, green chop and most importantly it's ability to draw pollinators. It's basically a bee factory.
I hope this helps!