Low voltage leads to higher current draw which is what is hard on the starter. Why not replace your battery bridge switch with one that isn't momentary contact?
Deep cycle batteries are not rated with CCA that is only for starting batteries so don't worry about it fwiw a 24 volt system will only have the CCA as the first 2 batteries you can add as many as you like it won't change the CCA same with a 12 volt system the 1st battery with the CCA rating never changes with the addition of batteries good luck