Should I aim for higher NP now or later?

You have pretty much answer your own question. If you wanted MHXA which is not far away, why wouldn't you save your current SQ for her? especially you said you don't buy SQ at all

The biggest jump in damage/performance in NP levels is at NP2. If you look closely at the numbers, the damage difference between NP2 and NP5 is the same as between NP1 and NP2. So that means that aiming for higher that NP2 isn't as good of an investment numbers wise. So, unless you're determined to get higher than NP2 for the sake of it, your SQ would be better spent elsewhere.

The gains from NP1 to NP2 is the biggest.
NP2 to NP5 is about the same, but you are not likely to ever get it on a 5* if you are F2P, or a dolphin (soft-whale), specially a limited one.

NP2 to NP3 is barely felt, NP2 + the Strenghtening Quest for NP is enough for double Skadi shanenigans wreck any wave.

As other said, the return is negligible, but keep in mind you could always pull for him again during next year's summer banner right around when she comes out for np3+. At that point, he'll have jumped up in the tier list already with his powerful interlude and it'll be more worth it.