From Above

Thanks. 

I'm still unsure about the solution I currently have but in a nutshell:

• I've got a level scene for each difficulty tier
• The level is separate in screen sized sections containing a list of enemy patterns to randomly choose from. You can also have several at once.

Pros:

• easy to setup and change

• you can somewhat tailor the level a bit better and avoid things that feels way too random

Cons:

• gives too random results anyway, sometimes ending sending all the hardest choices all at once, making it hard to properly balance 

There might be better ways to set this up I guess:

• Picking from a "deck" of options to avoid repetition 
• Favoring next / parallel options that fits well together 
• Assigning a score to each pattern to make sure to keep the difficulty balanced
• Etc...

At least that's a few things I can think of about this particular case but it eventually all comes down to the experience you want to achieve.