Sadly Nightmare Sphere 0 was never completed (or was it just lacking a save system ?). This one was very good, alas a little confusing at times on what to do, where to go. Has 5 endings if i recall correctly, depending on soul meter and status.
As for the metroidvania term, honestly i always jump to the conclusion of "The most popular thing will be the name of the genre". Metroidlike becomming metroidvania (recently heard IGAvania for all games created by IGA that are metroidlike), Wolfenstein being overshadowed by doom for doomlike (Then again call of duty-like is also a thing), Soulslike (when the concept of cryptic progression was allready introduced in the king's field series, or if you want to be a nerd, was allready a thing on tower of druaga on arcade).
Goonervania was a funny term to read, kudos.
Soulslike isn't about cryptic progression though. Well, mostly not. It's about the somewhat unique combination of skill based combat as opposed to stat based, using the players build and weapon choice as a hidden difficulty slider, high quality worldbuilding and a plot that does not hold your hand or force you to understand in order to progress without actually letting go to the point of not being there like open world games do.
Honestly, if anything I'd say that the build thing is the key component more than anything. What makes a soulslike a soulslike more than anything else is the fact that rather than having your build and level define your progression they just act as a modifier applied to your personal skill.
You can't just grind your way out of a problem or overlevel and spoil the game for your self. But you absolutely can experiment with weapons and builds until you find a difficulty that consistently works for you.
And I think that is something most people, even veteran players, just don't get. At least not on an intellectual level. It's not that the game is hard. It's that the game is as hard as you want it to be, and no more or less. And it remains consistently so based off your choice of build.
In fact, I see that a lot of supposed soulslikes simply don't get it either. They copy the outward appearance of a souls game. They have a stamina bar and a combat system that supposedly tests your skill. But they don't have that granularity in build quality and thus difficulty. And they are worse off for it.