TwoSix
Everyone's literal second-favorite poster
Which is precisely why I'd say to fold them together at 7. Yea, it would be an unnecessary thing to do at levels 7-10, except for using the Attack action during an Action Surge to cast a spell. But ideally, it would be immediately available when third attack opens up at level 11, and there's no subclass feature there. So why not just give it early?Part of the point is that you're using up weapon attacks for a spell. You get the cantrip for 1 attack at 7th level (post-level 5), when it means even after casting the cantrip you still have an attack to spare. So the Improved War Magic should be at least level 11, so you can spend 2 attacks for a spell and still have an attack left over. Before then, it's no different than just using a magic action to cast a spell.
So no, don't fold them together, but yes, it should probably be at a much lower level than 18. Maybe 13. Unfortunately the subclass feature progression levels doesn't make that easy.
I mean, is there anyone who thinks the ability is so strong it needs to be at capstone levels?