Progression and Small Keys

[Velvet's Simp]

I've been sequence breaking the game as soon as I got enough tools for it (To be honest, just the Sun Greaves was enough for it to start). I've also been hoarding small keys since I'm totally lost (totally my fault, and I'm loving it).
Are small keys tied to progression, or just optional stuff? I've seen many locked doors, but I have 4 small keys in total, so my fear is missing something behind a locked door that is required for progression and/or required for another small key. I do believe it would be difficult to have such a situation, but since I got some upgrades out of order and maybe went to places out of order as well, it might happen. lol

[Talon]

They do generally unlock shortcuts that can make traversal much easier, along with many other optional stuff. I think the first small key is the only somewhat mandatory key in the game because it's intended to unlock a door out of the Dungeon, which is one of two exits you have access to at the start of the game.

Though the idea of navigating the Underbelly without the wall jump does not sound appealing to me. I can tolerate the dark rooms leading to it though. XD