That's super interesting, thank you Drumbum42 for the in-depth analysis! That's interesting to read, and you're right there are a lot of things that would go into making good illusions. I like the mechanic of "you'd need to see a spell/creature before you could plausibly replicate it as an illusion", reminds me of how changelings or druids from many editions need to witness a species before they can morph into it.
I sent your post to my DM so he can keep all that in mind (I was worried about being overpowered, being able to replicate any spell I fancy and even deal the appropriate damage). These are all good limitations to factor into my play.