It's the way with staves/polearms, sadly. Previously I've used the aeroflex lagging as it's made to fit the pipes perfectly, but it's softer than more ideal foams and tends to tear pretty easily, while being a right pain to sculpt. You'll also find that past ~1.4m, the weapon will be whippy as all heck, and that's far from ideal.
As an alternative, if you can source 15mm fibreglass rods (try RD1), they shouldn't flex nearly as much provided they're a decent density. Otherwise you can go for 3 smaller rods (8mm) stuck together. If Derek/Ryan have any experience with this approach, it would be well worth asking them to share it.
Ideally, even a long weapon should be light, so it's easy to pull blows, and still fairly rigid so you have good control over it.