Dead battery. Again.

A lot of inaccurate information here.

The important steps are a) the 12 V in an unloaded state; b) the voltage drop with load applied and with that load at what voltage it stabilises and c) what the voltage goes up to when the DC/DC comes online - i. e. when the car is in READY mode.

Step 6 - unloaded voltage. Anything 12 V or below indicates a fully discharged battery. A full charge is required.
Step 7 - Voltage with load*. A stable voltage anything above 11 V is not a problem for the car, but as above confirms battery needs charging. If you have fully charged the battery and you do not get a stabilised voltage above 12.3, indicates the battery is in poor condition.
Step 8 - Voltage supplied by the DC/DC voltage. Any voltage in the 13.2 - 14.4 indicates normal DC/DC operation. What the voltage indicates is related to the 12 V battery's SoC - a flat battery will show up to 14.4 V; a full battery will show 13.2 V. If less than 13.2 V or more than 14.4 V, it is likely there is a problem with the charging circuit of the DC/DC converter.

*As a minimum, the load should be whatever accessories normally are on plus the headlights on high.