Just to complicate things a bit, I found that my catch improved after I focused a bit on hand entry and extension. Because the latter two were shabby, I was not in a good position at the start of the catch and I really struggled to get an EVF and good elbow angle. Once I improved my initial hand and arm position, it was much easier to focus on the catch. That's the great challenge of swimming; it's all connected. Without external assistance, it can be very hard for one to know where to start.

Also, I don't know about others, but I really can only work on one thing at a time. I usually can't even focus on both arms. If I'm working on hand entry, for example, I might focus on the right hand for one length and the left for the return length. Or, I alternate the focus in each interval; 50 yards concentrating on right hand entry, rest, 50 yards focusing on left hand, repeat. I might work on multiple aspects of technique over the course of a workout, but on any given lap I can really only think about one, sometimes two, things. It's part of why my progress is slow.