Yes, I agree that the "unbalanced" issue is incorrect, that actually batteries in a series offer advantages that, at first, seems counter-intuitive. But, we are all, me too, inductive thinkers so we have experiences, just life anecdotes and no science behind it, and we later try to fit new information into our own "thinking."
You buy XXXX braid and for some reason it breaks off at a lure on your first cast? Most of us are done with it, forever, as if we have run a scientific study.
Most people understand that two 12V batteries don't give you 24 volts. An analogy? If you have two guys who get behind a car and they can, each one by themselves, push the car 10 MPH but their top end is also limited to 10 MPH, then two guys pushing a car can't push it 20 mph. They just are unable to move that fast. Batteries are like this.
What they can do is push the car at the 10 MPH for a longer period of time. Twice as long? No, not with batteries: it is actually much more.
Here is a great video by a battery expert dude explaining that with batteries, 1 + 1 is greater than 2. Worth looking at his vids to understand batteries better. It has to do with residual charge availability, getting more power out of batteries in a series.https://www.youtube.com/watch?v=snaJxB_psg8
Back to your situation: The only issue I can see is if for some reason when you use deep cycle batteries for cranking a hungry motor, that they lack the punch it takes. Deep cycle batteries are low on cranking power, high on long draining charges and having a series of them still can't pass through the power one might need.
But, batteries have a top end that usually greatly exceeds the cranking power to start a motor, especially a fresh battery, and you seem to be pulling enough power to get the motor started, so what's not to like?
I'd still replace the cranking battery especially if it has degraded with age or perhaps by letting it sit de-charged for too long.