Easiest way to create and maintain a custom theme?

I have made my own custom theme by

Is there a better way of doing this? Can it be done directly from my project without cloning the radzen-blazor repo?

The easiest way would be to use Radzen's Theme Editor (a paid feature). Otherwise you need to build the theme from the provided SCSS files as you are currently doing.

Is there any documentation how granular/deep the themes can be modified using Radzen's Theme Editor. E.g. visual apperance of (any?) Radzen blazor component?

No, we don't have such documentation. You can check the theme builder itself: