If you are going to keep replacements, you need to realize that you can't mate every cow to the "right" bull and get a uniform crop, you are only dealing with a small number of animals.
Therefore, you need a system that you can put in place without worrying about age overlap of different breed types of cattle. The cows will be there for 8-10 years or more, so the decisions you make today, you have to deal with for a long time.
If you strat messing with more than one breed and then change sires every 4 years or so, then you will have cows of several breed types spread out over 10 years.
Therefore, if you are going to sell all the calves, it doesn't matter what you do, but a Continental bull with best optimize the marbling and YG antagonisms.
If you are going to keep heifers, then there is only one thing to do with the small numbers you have.
You need to buy some sort of an Angus or Red Angus cross bull. Gelbviehx, or Simmentalx would be my picks.
In this system, you can trade for the othe next time and not lose uniformity. Buy at Gelbvieh/Angus bull this time and a Simmental/Angus bull next time. The Angus percentage remaining constant will keep the British/Continental percentages constant overtime, so you will not lose uniformity. Every calf will be 50% Angus, a little more since that is where you are starting from. I say Angus, I mean Angus, Red Angus, or some other British breed crossed with the Gelbvieh or Simmental.
If you pick bulls that have the same "look" regardless of the breed, you won't lose uniformity.
Don't worry too much about the folks that say you shouldn't use crossbred bulls. Most of them just have purebred bulls to sell you, or they run upgraded commercial herds, basically upgraded Angus of Hereford for the most part. In other words, don't listen to the ones who CAN'T run a crossbreeding system, listen to the ones who CAN.
The systems I have described above actually bring a little less heterosis to the system than if you used Angus and Gelbvieh or Simmental in a 2 breed rotation. But, the percentages of the breeds are constant over time, so you don't get the flipflop of differing biological types of cattle in the same herd.
Crossbred cattle are NOT more variable than purebreds, but a bad crossbreeding system can produce swings in biological type within herd that results in lots of different percentages of each breed being produced. This is mistaken for non-uniformity. Big difference.
Badlands