You should maybe add more characters to make it very hard, I thought about characters counting in roman numbers, or characters counting without some letters like without vowels (n to thr fr fv sx svn ght nn tn) , or without first and last letters (n w hre ou iv i eve igh in e) just to make it a real nightmare at one point (bcse I beat each level in one or two tries even at the end) , or a mode where they're counting faster. Otherwise, it's very good for a game jam.