mad Talking about the exact position of this feature, I think it could fit well as a snippet option too. The only way I firstly introduced my idea specifically in connection with custom blocks — is simplicity for the end user: custom blocks provide person exact what is needed (maybe some sites doesn't need some of default blocks) and they are easy to choose, not like a snippets, which shows all the snippets from themes at one time as the list without any icons or images.
Talking about the current solution, I can say that I personally usually just use static templates for these with a fixed amount of cards (or some other elements, you know, it is relevant not only to the cards), which makes me or end user less free.