In my opinion it's the pacing that lets you down in terms of making a metroidvania. Instead of a gradual increase in difficulty and power, it just feels like I've been thrown into the game with harsh difficulty right off the bat, which makes its pacing quite unmetroidvania like. I can definitely sympathize with that being part of not having enough time to add more rooms that make the pacing more gradual, but it does mean that the experience has a very awkward flow in my opinion. But I absolutely wouldn't say that you've "failed" in making a metroidvania at all, this game is ultimately a very competently made metroidvania, but it's more so that I believe this game would have actually been stronger if it wasn't a metroidvania with its pacing and design philosophy.
I definitely think more frequent checkpoints is a start in this regard, so it doesn't feel like the player is being pushed too hard by having to do these difficult rooms back to back right off the bat. I do have to stress though that this is still a very strong entry overall and I had a lot of fun playing it regardless