Open ended question - input field font should be consistent with other text.
Maybe adding classic checkbox with check mark instead of black fill. Same for radio buttons ( just a circle inside, like on other content types )
On desktop error msg red background should be smaller in height with significantly smaller font.
Text in general look bit over-sized on desktop view, specially error msg and buttons. Text sizes on mobile looks great!
Answers should be a bit smaller comparing to question text.
When answer is two or more lines it would look better if aligned with first line, like in attached example. Also, it would be good to add a bit space between answers (1.jpg)
Empty space above and below question should be reduced, this takes much space unnecessarily. Same goes for bottom black bar on desktop. (2.jpg)
Suggestion: Progress bar at the top could be animated as in personality quiz.
Bottom black color behind buttons should be #1f1f1f.
Bottom black color does not work good when content type is on dark background. I suggest adding border at this area - same color as buttons border. (3.jpg)