Lol i think nowadays a lot of discussion is actually on his character being rather selfish. He was also one of the main figures who advocate to surrender to imperial court which lead to almost everyone dying including himself being poisoned
On hindsight, with the talent at song jiang disposals, he could have just rebel and start a small kingdom. Like today, where many ppl start their own business after resigning from company.
But song Jiang is not a risk taker. He want to jiak govt ricebowl. And it so just happend that song dynasty is a unique dynasty where the emperor like to pay $$$ or give out benefits to avoid war or conflict.
So from his view, making a name as a rebel leader then being offer a govt position is a great deal.