@WolfCreek, welcome to Cattle Today! You are getting some excellent suggestions here, but I'd like to add some 'why' to these. I don't know what your soil tests show for nutrient availability, but correcting the pH before considering any actual fertilization is important. The reason being, is that when the pH is 'off', nutrients that are in the soil aren't as readily available to the forage grasses even when present or in worse cases, may even be present in the soil and not be reflected at actual levels in your soil test. Additionally, if you add nutrients to a soil with a skewed pH, the nutrients you have added very well might not even be accessible by the forages due to the pH.
You indicate that you rotational graze and I appreciate that. Many producers know that rotational grazing is the right thing to do, but they don't realize there is 'minimum' correct way to rotational graze and a whole lot of different 'incorrect' ways to rotational graze. I'd encourage you to check out the threads here on "The four Never Fail Rules of Grazing" or Google the same. Proper rotational grazing will promote root growth. Roots on grass plants are completely replaced after two years. As a result, all the roots you see on a grass plant will be "converted" to below ground organic matter two years from now. Converting roots (if you have a lot of them and proper rotational grazing will get you a lot of them) to organic matter is much better than adding hay on the surface of the soil for organic matter, that doesn't get incorporated into the soil. Don't think tilling the soil will get the OM from the surface to mix into the soil. More organic matter is actually destroyed than is mixed in if you do this.
Genetics is key when grass feeding/finishing, and frame size is a big part of it. Bigger framed cattle have a more difficult time grass finishing than smaller framed cattle as a rule, but it's not a perfect correlation, hence why you want to look at the genetics.
You might consider some annual forages for grazing that might contribute to a possible faster marbling rate such as oats or some legumes. However, I would suspect you will get a faster improvement on finishing animals by focusing on their genetics.