what's the secret to making a good robot in robocode

I think of "assume non-orbital movement in Melee" and "assume orbital movement in 1v1" as two sides of the same coin. Either way it's an assumption. You could go somewhere in the middle by having both GF-based and PIF-based (and displacement-vector-based) methods and picking the one that seems to best characterize the current enemy, but even then you still make an assumption: That the assumption of a fixed method is suboptimal often enough that it's worth the penalty of sometimes picking the wrong one ;)

In practice, they are two sides of the same coin, but I still disagree. Only the "assume non-orbital movement" involves extrapolating enemy movements (relative to anything) and translating to and from firing angles. Waves collect very close to the exact value that the Robocode API takes as input to decide if you hit an enemy. That just doesn't feel like adding any assumption that isn't already introduced by Robocode itself.