Finished this today, with 151 of 175 strawberries; enough to unlock the best ending (which requires 150).
Never used assist mode, but it did take me nearly 20 hours and just under 3000 deaths. I've also unlocked a post game level with the crystal hearts I collected, but I haven't played more than a few minutes of that yet.
Overall, I thought the game was absolutely brilliant, but I did think the story and writing took a very significant nosedive once Madeline and "Evil" Madeline teamed up. The entire last level is full of trite, schmaltzy dialogue with all the subtlety of a sledgehammer, and I think it severely undercuts the theme of everything that went before. What sort of meaningless shite is it to say that depression, anxiety and chronic self doubt make you better as a person? Surely the message should be that in spite of your mental health problems, you can still achieve and overcome seemingly insummountable odds. Not because of them.
I just found that really disappointing, shallow and borderline obnoxious, to be honest - even if I did like the writing before that, and in particular, the characterisation of Madeline.
Great game, though. Excellent even.