Two-photon grayscale lithography (2GL) is the latest evolution in two-photon polymerization laser 3D printing, which combines unmatched surface quality and high throughput enabled by continuous laser power modulation, that dynamically controls the print feature size (voxel) to smoothly blend contours despite coarse slicing. However, to reconstruct shapes, 2GL solves an inverse problem requiring correct parametrization of the voxel size and their underlying print parameters. Such parametrization is specific to a given feedstock resist and so far, only available for a few commercial polymers. This paper establishes a 2GL 3D-printing route for the rapid manufacturing of complex silica glass micro- and nanostructures of unprecedented quality and shape accuracy, substantially advancing glass micro-fabrication. This is enabled by a pre-glass polyhedral oligomeric silsesquioxane (POSS) resist, engineered for high polymerization sensitivity and dynamic range. Together with a routine to identify optimal grayscale parameters for such custom resists, we realize use of the 2GL process at maximal laser focus scan velocities while maintaining a high loading of the silicon-oxygen POSS-cluster source. Mechanical resonators with quality factors beyond 1000, as well as optical components with <3 nm roughness and nanoscale diffractive patterns are demonstrated, opening new pathways in optics and photonics, precision mechanics, and metamaterials. Finally, the introduced routine to identify 2GL parameters readily translates to other resists, providing a framework for extending 2GL-printing to a broader range of materials.