I use 4ga for all of my battery connections (parallel or series). For running to your trolling motor, how are you going to connect it? I am using stud connectors, and I'm also running 4ga from the battery to that terminal, then attaching whatever wire came off the trolling to that same stud.
If you look at the bus bars here - I'm using the single stud. They also have another one where you can hook up smaller accessories and get power, I just didn't see it when I ordered them at first -
https://shop.genuinedealz.com/Marine Electrical Supply/Busbars and Covers/. This website is where I get all of my electrical wires and stuff from - hard to beat on price....
For any lights, fish finders, bilges, livewell pumps, etc - I would use 16-14ga. 10 ga is a little too much to connect those all together with. What comes on all of those accessories is usually 18-22ga wire, and it's hard enough connecting 14ga to those tiny wires.... even more so if you used 10ga.