So first of all, you can cycle the really difficult goals by changing goals in the class menu for 30 mana. Besides, being mindful of your squad's clothes durability also helps, since they can only equip when the clothes are broken.
Secondly, i'm not aware of what you mean by selling loot instantly, the gems are a more efficient way to carry money in dungeon since one gold stack caps at 1750 (to put in perspective a stack of five 500gold gems values up to 2500, allowing you to bring back 750 more gold than regular gold would have, and it gets better with more valuable gems). So i don't get the idea there. Taking back seemingly useless equipment also helps, since for each 4 item of the same thing, they become an infinite stack and stop appearing in the loot pool, same for cursed items (though, you do need uncurse them first).
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Hmm not sure as i don't see it or not but the quest rewards a ring and i have no way or more specifically speaking where is the inventory to switch gears after the quest? (main problem to my complaint since i am given the illusion that all my gear/potion/healing stuff is sold/converted) I must have missed the changing goals since i usually spend circle/mana trying to find where to change the goal itself by buying more shops upgrades and so on. Also will we get to grapple enemies in the future or only us are the targets of grapple. Overall this would have been a game i enjoy but minor details bother me a lot and not enough adult content at the moment to garner further interest.
Rings and accessories alike are equipped on the last three slots, even the reddened slots (the colour is just used to indicate an empty slot, i think), you can just click on them, or an existing equipment and select an accessory.
Regarding the more grapple enemy idea, they added a slime that can do that in the patreon version, which will come to public eventually (as far as im aware of). So i guess yes?