Congrats on a successful plunge into cranking. Once started you either can or can't live without it. Personally, I can't. On your battery issue. If you can invest the funds, use group 31 batteries. I use Trojan batteries but they do hit the wallet hard. To extend additional time on the water you can install a second set of batteries using an additional wiring and female trolling motor plug. This allows you to unplug the set that are winding down and plug in a fresh bank of batteries. This is what I did and enjoy having two banks.
Other options involve utilizing windsocks or five gallon buckets, drilled with holes, to slow the big motor down. Others do use a type of trolling plate on their big motor. Some here actually troll backward with big motor to slow down speed. However, I have enough challenges going forward to attempt walking backwards. Lol.
Finally, on your concerns about the trebles on the cranks. You can file/cut the barbs or change out the hooks to barbless. You could also just remove one set. Hope this helps and you decide to continue cranking cause it's a hoot.