What is the bed surface? Some droop is normal and what you have is pretty tiny in the FDM realm. Leveling should be done over the leveling adjusters and it is not uncommon to have high and low spots on the bed also. Especially if it is the stock bed without glass.
PETG does not like to be squished like PLA does. I use a glass bed and glue stick for PETG and it has worked nicely. I give a good level with paper and use a first layer height of 100-110% I think.