Last week I tried Astra after months of procrastination and I found it really good. Just wonder that I should have used it before. So I am planning to implement it number of my sites. It is a fast, lightweight and very much customizable to fit my needs. You can give it a try.
Yes, customization has been one of the strongest fortes of the Astra theme. Generatepress is awesome and you can build any type of site you want with it. But customization-wise Astra has the upper hand over GP.
For example, in the base GP theme without GenerateBlocks, there are limited things that you can do with the headers. On the other hand, without using any extra block features, Astra does have more features. It has 3 separate bars called the Above header, Primary header and below the header and we can place elements to left, right, and in the middle. We also have options to drag the required element wherever we want. We can control each of the bars settings such as colors, border, height, width, and a lot of other options.
The same goes for the footer as well. In GP we have limited options but with the Astra theme, we get three bars called Above footers, Primary footer, and below the footer. We can customize them the way we want.
That's why customization-wise, Astra has more features. But Generatepress also gives complete control to customize the site but with more manual controlling rather than giving lots of options to play with like Astra theme.