Ok, so after grinding for what felt like an eternity, this is a compilation of what I've found. Not sure if everything is brand new, but it should be. Honestly it's a far more substantial update than people are giving it credit for, its just buried within a lot of grinding and no direction. You probably should double check if you have riming enabled in "fetishes". This was a no sharing playthrough, keep that in mind.
Ok, so assuming the Onsen event triggered for you, after that, the first thing that should unlock is a scene with Diana in the bathroom
At the Mall, you can buy a new sport outfit For Diana. Doesnt unlock any lewds, but unlocks new Yoga positions. Giving it to her also unlocks one of the scenes in the gallery
You should also buy a dildo in the dark street. Unlocks a new option for Amelia Massage and a new option for Diana at the Onsen later on.
After working out with Diana, you should have more options in the shower. I think the BJ is new, but the gallery only unlocked for me after the lv2 fingering scene. If these are not avaliable, you probably need to repeat the Onsen or shower a couple times. But we'll get back to the shower.
Ok, so the repeatable Onsen scene is EXTREMELY grindy, and I'm pretty sure its dependant on her affection and submissive level. I did it with max affection and negative submissiveness. The last scene you unlock should be the dildo scene, assuming you bought it. Again, remember to have riming on. You should have these:
At this point, you should have Diana's gallery unlocked, but after finishing the Onsen anal antics, theres more in the shower. Anal fingering and ass humping:
If I've missed anything, let me know. Still missing 3 Amelia Gallery scenes, if anyone got them unlocked, help a guy out.
EDIT:
Nevermind, found the Amelia scenes. Theres's a button that pops up while you're massaging her in the kitchen that I've never noticed because I just skipped seen text. Click that, theres some breast fondling and another button for anal fingering after. Then, yet another button for putting shit up her ass. LV1 is Cob with condom (needs to buy condoms), Lv 2 is cob without condom, LV3 is vibrator (needs to buy from Dark street iirc), LV4 is eggplant, LV5 is eggs. Don't know what the affection and submission requirements are. Did it with max affection, -15 submission. Yes, more grind.