So I think it would be an obvious improvement and might well be worth an increase in the resulting master public key size just for additional robustness, I don't know that in practise it would safely permit intentional use of it.
To say that public key size is the problem here seems kind of vague, I think that the main deficiency is that you need to perform m elliptic curve exponentations to derive the next pubkey, instead of a single exponentation. So I'm not sure if the tradeoff between the extra complexity versus the supposed better security makes sense (with non-hierarchical variant), it depends on whether the security improvement is significant in practical scenarios.
You use multi-exp which is not N times slower, and wnaf with some big tables on each of your points, so it's only a couple adds even if your coefficients are big. libsecp256k1 on a fast laptop does something like 70k ecdsa verifies per second, and that involves a multiexp on two points (and a number of other expensive operations: a modinv for s and a sqrt to recover the pubkey). So, I don't see why you'd consider that an issue even with hundreds of points. The reason I cited size is that the advantage of the homomorphic derivation over independent keys at all is size, and having to grow the pubkey linearly in use (to be secure in the worst case) erodes that improvement.