We've been in the Simmental business for over 40 years, and we started by AIing commercial cows just like the cow you have. All were small to medium framed black, black baldy, and red baldy cows that needed more grow and more muscle. Simmental is a perfect match on cows like that, and I think you'll like the cross.
A few purebred bulls I would consider would be Upgrade, Triple C Singletary, or maybe TNT Tanker. All homozygous black bulls that are also homozygous polled, so you'll get a black or black baldy calf (since your cow is a baldy) with no horns. Those bulls are all a little different, but they'll all add pounds and thickness without extreme birth weights.
Personally, I wouldn't use a heterozygous black bull in that situation unless you really don't care if you get a red calf. The chances the calf will be red are 25% or 1 in 4. That's not a low percentage. I wouldn't necessarily worry about a lot of white, but if you use a baldy Simmie bull, be prepared for some extra chrome like white belly, white legs, and white tail. You can't really predict the chances you'll get some white, but it's pretty good odds.
