Given that I have no intentions to shoot videos, is there any reason to choose the higher speed (e.g.: Delkin Black vs, standard) cards in my Z9. There is a significant price differential and I am not sure that this would be money well spent.
You shouldn't have any problem with just photos. I made a post recently just because i have a Z6iii and just like a Z8 it sandwiches a CFe+SD card on top of each other. The Z9 has no issue as they are side by side.
For non-video. Buying whatever is cheaper at the moment would work for you in any case. And at the moment. Some newer faster cards than old stock are the cheaper option.
So when it comes down to speed. Video cares about speed because of the long term Minimum SUSTAINED Write Speed. Not just Minimum write speed. While Photos unless you're doing buffer clearing Action/Sports shooting. This spec to look at isn't that much required.
And also as mention i've been reading that the OLD delkin Power and certain capacity Black cards are much cooler. I'm not so sure about the Delkin Power G4 series. As they are electronically different than the old Power. But heat might not be an issue for you.
For the kind of simple shooting I do, it doesn't require a CFe card, but the extra speed is nice at the end of the day when I'm unloading a full CFe card onto my PC.