Suggestions for difficulty balance: the challenges feels more like a puzzle, where when you find the correct build and/or gameplay style, they became quite easy and you are able to complete them consistently. The problem is the price to figure them out is to high. Give us a practice mode for them. Maybe the goddess want to mess up one with another, and one let us practice the challenge of the other, without costs neither rewards. Maybe in the training there is less balls to collect and less bits to kill, so the challenge still feels like a challenge when we try to complete it after practicing a bit.
For my experience, "bit magnet" is the most essential card; but that's not intuitive. I only tried it farm bits more quickly, but it soon became obvious it is need for max level and even for one of the challenges. I think collecting bits for reducing the time to get the balls is very important, but actually collecting the bits in the chaos of the battle is overwhelming, and probably leaved behind for many players. I would guess that buffing a little bit the base radius of bit collecting (without the card) would be a discreet change that would make the game easier to reach max level, without changing to much the game fundamentals, neither feeling like the dev is taking you by the hand.
I also think the challenges should feel more optional (and I am certain they will feel more optional already when they aren't the novelty of the month anymore). While in my opinion the private rooms are good optional content and are fine as a reward for the challenges, there is an argument to be made that porn should be easily accessible in a porn game. And the surges does not feel like optional content imo, they feel like a upgrade that we should be able to get way earlier. A upgrade that could potentially make the game easier shouldn't be a reward for those who already think the game is easy enough to reach max level; there is no impact for those people, while is missing for those that are still founding it hard to progress. What I think feels way more like a optional content would be some (much needed) character customization option. Some new colors or painting designs that are locked behind a difficulty challenge, to be wielded only by those who deserve it. Maybe even rescue that old idea for masks...?
And, of course, I will repeat once more: upgrading a character or a card should be less grindy. There is no need for the game to be so much grinding, because the game is fun, it will already capture players without the grinding. With some math (~250 cards, 10 for max level, ~12 cards (3 packs) each 80 minutes (8 runs, a day reset)), you need at least 280 hours to max the level of all cards. You don't need to max the level of every single card, but you probably will not max out not even the ones you want because of RNG. Give us more cards per minutes, and give us less RNG-dependent ways to upgrade the cards. Getting good cards is absolutely necessary for max level and for challenges, and I would bet many of the players complaining about difficulty do not have the necessary cards, because the grind is this much. Less grind is more players meeting the requirements, and less complaints.
If the IA of the dolls were a little better, the game would also feel better, but I guess thats a whole different beast, not so easy to adjust; but maybe this other changes would not be so hard.
Anyway, I still think this update was great, I love the new challenges and for me (~40 hours of gameplay on version 6) they weren't that hard at all, and were extremely welcome. Those suggestions are only because I'm seeing the backlash of the challenges and you said you want to re-balance the difficulty, so why not give some suggestions. I hope I'm not being to invasive, and I trust the work of someone who made such a great (although not without some problems) game.