This is way cool. You're basically just extending the words' length the longer you get, right? And variate it a bit. Mostly it was the words that were new to me or words I don't normally would use that were the hardest to spell, it would be cool if you ever made a new one to have many more factors play into how it finds the words, such as the amount of vowels vs. consonants in a sentence, the harder to "pronnounce" the harder I imagine it being to spell in your head
I looked online and found a word list containing about 60000 words, one word on each line. I used visual studio to do a huge Find-Replace to format the words into an array so that it can be accessed just like you would out of a normal array, and picked random words out of the array.
Maybe keep the too easy for 10-20 words, then a bit faster at 30, then at 50, then at 100, then 200, then 300, then see how long they can keep going with it getting faster every 10 seconds after that .3.