How does nothing in between make sense??
Wouldn't a 2-3 year bridge deal make the most sense especially given some of the points you make here?
You sign him to a 3 year, $4-$5 million deal and at least lock him up for the next 3 years. That way, if he continues to get better, you have him signed to a reasonable cap hit and can re-evaluate a longer term deal at the end of the 3 years since he'll still be an RFA.
To me, that's really the only option. Sure, you could sign him long term but then how much money is that contract?? And what happens if he stagnates or runs into injury issues? And a 1 year deal doesn't really make sense at all IMO because as you said, if he breaks out you just have to worry about signing him next offseason.