>>933840Paddling when fishing does sound kinda nice where you're not allowed to anchor and there's lots of current, but if you're either anchored, trolling or drifting it'll be nothing that a small anchor can't do desu.
Does you upper body get fatigued normally? it shouldn't really unless you have a paddle with very large fins or you paddle with your arms (disregarding conditions, that is)
Getting a new kayak is great and you should if you can afford it, but it might not be mush of an upgrade if it's only for fishing and doesn't improve much upon what you already have.
If you're buying a new kayak because you want to camp, tour or cross channels and other more ambitious activities, I'd recommend it whole heartedly.
Try a few out if you can, as well as smaller bladed touring paddles; you'd be surprised at the difference that a new paddle can make by itself to your handling and efficiency on the water.