You should be able to see the main shows without much problem. Service could be with different waiters each night, and I find that to be less desirable.
7:30 is a perfect dining time though, to my thinking it is ideal since I think 6pm is too early and 8pm too late.
If it were me, I'd stick with a set dining time to begin with. It might be possible to switch to "my time" dining later in your cruise, if you choose to do so.