how to patch a string of lights, single channel

There is no specific fixture that would display like this. You would need to create a part for each light source in patch and move it to the appropriate xyz-position. Since these additional parts only exist to help Augment3d, these additional parts don't need any DMX address.

Maybe this support article might help you: https://support.etcconnect.com/ETC/Consoles/Augment3d/How_Do_You_Duplicate_a_Fixture_in_Augment3d

Do you have a suggested fixture you would use for a single light that displays appropriately in A3D?

Do you have a suggested fixture you would use for a single light that displays appropriately in A3D?

Probably the closest you can find is the Astera Nyx Bulb.