(A faulty battery wrecks other good parallel batteries. And AGMs can go into thermal runaway. And "n" batteries means n times the failure rate (where failure rate is the individual battery's "self" failure rate).)
The "more load" is CRAP. AH is AH - it doesn't matter how - end of story.
And the more load is crap anyhow. I won't repeat my writings elsewhere, but if someone is keen to explain how reducing the size of your main battery reduces alternator "strain"....
(It doesn't - in fact the opposite, though strain is not the right word. A bigger battery reduces the overall current that an alternator needs to supply.)
And if someone wishes to justify otherwise, ignore cranking from the battery-size equation.