I think you can do both, that is breed your better Angus cows up to PB status and use embryos in the bottom half of your herd. When I had our Charolais herd, I got my foundation cows from a neighbor who had started with a very good Angus cow herd and used the best Charolais bulls he could find. He also had a very good AI program for many years, and when I bought his herd, he had been at this for over 20 years. The end result was a set of PB Charolais cows that were very unique in the Charolais breed. They were more moderate framed, easier fleshing, and superb uddered. Most of the year they looked white but there were a few of the PB Charolais cows that still had a grey tinge to their hide, when they shed out. It will take a long time and a lot of commitment to breed your herd up to PB status.
I also have done something similar in that a very good cow family in my herd, is bred up to PB status from a very good Simmental X Angus cow I had in the 70s. This really wasn't planned but got started when one of our herd sires decided to visit the pasture this half blood cow was in. We tried to abort the calf a few times as we were really wanting a 3/4 Simmental calf but she wanted to keep this baby and I finally decided I better let her have it. She had an incredible Shorthorn heifer calf and I kept her in my herd. I got really lucky as every female in the next generation produced a heifer calf early in her life so I was able to move up to PB status about as fast as it is possible. Some of the females from this line have been high sellers in some of our production sales.