Symmetrize Texture
Why can't I change the viewing angle while using the 3D brush?
This comes from the specification related to Blender add-on interface (called 'Modal Operator'). So you need to exit the brush operation to change the viewing angle in the 3D view.
What types of image are supported?
Basic image formats are supported: jpg, png, psd, tga, bmp, and gif.
Tiled image (UDIM) is currently not supported.