This setup has some questions.
To begin with, it is always best to have a matching set of batteries.
It is also wise to have them charge at the same rate.
By your description you have neither.
I have a pair of group 34 yellow tops in a dual configuration that are coupled when in the ignition is in the run / start .
Both batteries work to start the engine and charge together thus having as close to the same initial current draw / drain and charge.
I’ve had this setup with the original batteries for 10 years so far with no issues.
In your setup, you always draw down the starter battery but your secondary is always topped off.
I do not understand why you have a 7 second delay.
The issue with that is the charging system is working to recharge the starter battery while also overcharging the secondary battery.
Another possible issue is if your vehicle has a “smart” charging setup from the factory.
These smart charging systems do not fully charge up AGM batteries.
In general a dual battery setup is to provide power via the secondary battery for acc. so as to not deplete the starting battery.
What are you doing with a Cadillac DTS that requires such a setup other than perhaps a stereo?