The 40 shards requirement in a mistake, it will change back to 20. While I agree that the hell mode of later levels is significantly harder, you can still get them (and not specifically at a much lower rate) in hero mode.
And once you are done with the levels, you shouldn't replay them manually, you are swimming in skip tickets if you roll the money gacha a few times.
Edit: Yes, there is still gems in the star chests.