I do have a couple ideas of how I could preserve the early game - mostly just things I realized I designed wrong. It is kind of weird how the game can feel so different in the early game than even the mid game. Just the lack of choices in placement early makes a big difference in the game feel.
Some things I've already learned about the director is you need multiple enemy types to make it works. Spawning enemies in a different pattern like I did is a poor substitute. And that the idea of having multiple directors running in parallel is probably the right way to go. I noticed that the line spawns almost feel like a separate director because the enemies take so long to get to you so they start to overlap with other spawns and arrive at the same time. Multiple groups of enemies arriving at overlapping times is quite compelling.