Here's a suggestion.
Create a new gamemode which combines the waves mode with the sandbox mode.
It would make you try and find the resources to build everything but you could choose when the waves come.
Or just add a world generation menu to make players able to change settings of the world so that we could make waves spawn less often
This suggestion is because your game is great, and I like it A LOT!! But it is also hard (which is great), so new players can play a "Normal" game but easier. It would be something like a training mode.
Anyways hope you read this, keep up the great work, you got a fan here!