I suppose in most teams, including the Sharks, there is not much difference between a half back and a 5/8. The better of your halves at passing and kicking tends to do most the passing and kicking, and the better half at running takes on the line more. They swap in and out of first and second receiver, and swap over sides of the ruck (e.g. Lockyer is often first receiver for Broncos and does a lot of their short kicking, though Wallace is half back). Halves go wherever the coach wants them to in defense nowadays (e.g. Preston going to the wing in defense when the Panthers had him) but you'd like to rely on your 5/8 as a defender more than your half back.
Having said that, Seymour is better at tackling than most halves, which would more traditionally make him a 5/8 than a halfback, but has a reasonable short kicking game which would more traditionally make him a half back.
I think he has done well for us, but would be better if he got his hands on the ball more often in attack, and Bird is so hard to stop running down the blind side, I would prefer Seymour as halback and Bird as 5/8 for us next year.