I'm more familiar with EE levels. From OPM: https://www.opm.gov/policy-data-oversight/classification-qua...

and if you can find: POSITION CLASSIFICATION STANDARD FOR ELECTRICAL ENGINEERING SERIES, GS-0850 ELECTRONICS ENGINEERING SERIES, GS-0855 TS-3 February 1971 It has good information on what level engineers are working.

I think you have the correct frame of reference, senior level positions are those who are used to handling projects/questions without guidance from others (although most senior folks are willing to ask questions to learn)