I'd suggest going to the Smartmix calculator on the Green Cover Seed website
https://smartmix.greencoverseed.com
Smartmix calculator is a great tool. It will ask you a few basics like your zip code, planting method, planting date, acres to plant, etc.
Then it asks you to prioritize 3 goals. (building OM is one of the goals to choose from).
It then allows you to choose from lists of a variety of seeds.
As you plug in different varieties, the calculator "rates" several outcomes, based on your stand goals. It also rates the C-N ratio, and also rates the potential effects of the mix...stuff like nitrogen fixing, grazing, drought and frost tolerances, and a few others.
It also advises seeding rates, and price to deliver it to your door.
BE WARNED...Smartmix Calculator is addictive. You can spend hours playing with different mixes. And be prepared to want to try more stuff that you probably have space to plant it.