It is true that thrusters require high amps discharge rate, and if used very seldom and briefly, only discharge a very tiny AH amount like starter motors.
However batteries designed for deep cycling - if of sufficient AH capacity - have **zero** challenge with that load pattern, the thin plate design of Starter batts is only an advantage with undersized banks.
And sometimes, thrusters do get used for more than a second or two. If such usage is regular, then a deep cycle thicker plate design is required for bank longevity.
Some examples, just using a couple (AGM) batteries I'm familiar with:
Odyssey PC-2150 (G31): HCA = 1545, MCA = 1370, CCA = 1150
Lifeline GPL-4CT (GC2): HCA = 1085, MCA = 925, CA = 750
I think I remember cranking amps for multiple batteries in a bank are calculated similarly to the way capacity (amp-hours) works. If so, cranking amps remain the same when 6Vs are series-paired into 12V or 24V or when two 12Vs are series-paired to 24V, and added when two 12Vs are parallel-paired, etc.
So a pair of PC-2150s at 24V would give HCA = 1545, MCA = 1370, CCA = 1150. (Or at 12V, a pair of these would give HCA = 3090, MCA = 2740, CCA = 2300.)
And
four 4CTs at 24V would give HCA = 1085, MCA = 925, CA = 750
Is that math/concept correct?
If so, maybe it comes down to a "highest cranking amps versus fewest batteries (cost and weight)" comparison.
I remember discussing my current set-up with Lifeline ref whether the 4CTs would be sufficient for starting one of our diesels... minimum MCA 1560 and minimum CCA 1250 at 12V... and the answer was yes, as long as we had at least 4 of them in the bank...
I dunno how much use thrusters actually get (given we don't have 'em) and it probably varies widely by user anyway... but the difference in overall lifespan for a starting versus deep cycle bank may be 9 years versus 10 years (or whatever, for example, and using OP's 9 year estimate), and I wonder if one could really pick which of those might be longer.
-Chris