If we're talking about gear purchase choices I have some observations as well:
I definitely agree that Buttplugs are by far the most generically good purchase you can make for your party, and they're highly affordable as well. They're virtually required to make Yarra a viable party member most of the time, and they help Riala a ton as well. I agree that for the early game the combination of Dildo + Handcuffs on Qum is arguably better, as that Lust Stun can be quite nice and it gives her something to do when there's no healing needed. As the game goes on I found that the odds of Qum having nothing to do on a turn started to go down and I transitioned to a more normal Whip + Buttplug setup for better magic attack for better healing.
For other purchases, I bought a Durge Sword that I passed around the party's various sword users for quite a long while before it ran out its usefulness, pretty good value. A Durge Axe for Hilstara can be a good pickup since there's very few good free axes out there and it's a looooong time before you find anything available that's better than Durge.
I also definitely agree that a Freezing and Shocking Staff with the discount from the Stineford magic store is great value. The party has a lot of spellcasters and there's a dearth of good mage weapons for them available for free. The elemental resistances can hilariously break a few fights too.
I actually did buy a Sylvan Sword for Simon when it first became available and I'm still using it in current content, so that wound up being pretty solid value. Simon's defense is pretty solid so it can take the hit, and it helps cover for his slightly anemic offense and speed. Some people like Sylvan Dagger for Aka, but I find she's too squishy to want to compromise her defense any more, and she doesn't need the speed, so it's kind of a waste. I'm still using a Steel dagger alongside her Masterwork one, and at this point if I replace it at all it'll probably be with a Sublime one. The Ardan Dirk is an interesting option, but I think it lacks the boosted crit rate of normal daggers, so that's scared me off of it a bit. I'll have to look into that more.
I also bought two Rune Shields in the Yhilin Slums store, and those got used for quite a while. They were probably worth it, but I could have done without. I also bought an Arcane Glove for Uyae to go with the free Zirantian Claw, but I'm still torn on if that was actually a good purchase or not. It makes her a considerably better emergency healer without compromising her offense too badly, but it isn't great value for the high price. As an aside, Uyae is really due for a gear upgrade. Her absolute best weapons are still from Chapter 3. I get that "Monk" type characters generally have bad gear selection, but c'mon.