Ok quick guide to maximum lewdness + saves every step of the way in case someone wants to skip grinding or whatever
1. Activating the second crystal causes the snowy mountain quest to appear in the guild. Completing this is also required for Swapping Play on the succubus statue before the THIRD boss, not second like some people in this thread say.
2. Activating the THIRD crystal causes the Bandit Extermination Quest to appear. If not, do the snowy mountain quest.
3. Once you do the bandit extermination quest, REPORT it to the guild and the next quest in the questline will appear. Each quest has cutscenes before and after completing so you might have to return to the guild on the next day to report it, and it's easy to forget this step. Once you report the ghost extermination quest in Fort Ray, Bandit Extermination 2 will appear.
This was all done with minimal lewdness or grinding, so there's truly no other requirements to do those.
SARIA'S INTEREST IN SEX
Saria's interest in sex is not strictly tied to her sexual stats. Rather, there's two events you must activate that each increase it by one stage. She starts with a "Passive" interest, then becomes Interested and then Lewd.
The first Lewdness event seems to require 100 lewdness, 30+ people experienced and the snowy mountain quest done. Then, make sure Rodrigo is NOT fucking saria and enter the dungeon with her, then use the return stone.
The second event is not tied to stats at all, you just have to complete Bandit Extermination quest 2 and watch all the cutscenes, THEN enter the dungeon and use the return stone once, which will cause the second event to appear.
If you do Bandit Extermination 2 while Saria's still passive, she'll only advance to Interested, and not Lewd. I'm not sure how, if possible, you can advance to Lewd if you do this, so it's highly suggested you farm for the first Lewdness event before doing the second Bandit Extermination Quest.
If you're like me and are mainly interested in the "Undressing" feature of the game, this is directly tied to her sexual interest. Once she stops being Passive, examine the table in your room to select Undress, and then beg her for it. She'll reluctantly undress and also cover herself with her arms, but is fully playable this way. If she's in a lewd state, then she's no longer reluctant and also stops covering herself. NPC reactions and dialogue also change.
None of these saves are a "complete" saves. They're just different points along this quest line that you can start to play from and still gather H scenes (and tbh these are for me if I ever need these again). It took a while to figure this out, but at least it's not RNG reliant like in Alchemy Fantasy R.