I think the key to holding onto Galvin is going to be the relationship he develops with Luai, both on and off the field. If Luai can take him under his wing, and be almost a big brother to him, as well as helping to improve his game, that's going to go a long way.
As long as Galvin is also open to playing a somewhat lesser role next year. He's basically had to everything this year which is a massive ask for anyone let alone an 18/19 year old. With Luai at the helm he'll be able to sit back and pick and choose his moments a bit more, which I reckon will be good for his game. Being a young bloke on the way up, he might not want that and that might cause a problem though, so Marshall and the other coaches will have to monitor that closely.
Aside from the length contract, what does everyone reckon would be a reasonable offer $ wise. I know it's hard to say as he has his second year ahead and he could improve out of sight, but for mine if he stayed at his current level, or even improved slightly, $750K to $800K seems about right. I'm not saying that's what we'd end up paying, just what I think fair value is.