All right almost a month later and I've now finished Samus Returns. 98.7% completion which I was pretty proud of, took me around 18 hours according to the game, but the 3DS activity log has it closer to 25 hours. So not sure if it doesn't measure deaths, or if I just spent a lot of time gawking at the pause screen/map.
Either way, the game definitely grew on me, Samus gets really nimble and especially when you have the screw attack going it's pretty fun to blaze through areas. I still miss the sparkshine ability, though I can see how all the verticality in this Metroid game would render it fairly moot. Spiderball makes up for it, but I was kinda hoping it'd get a speedboost at some point.
The power bomb took a long time for me btw, which is a real shame because there's a fun trick you can do with it: spiderball onto a surface and then launch yourself with the power bomb to rocket across the room.
Gripes: storywise this one is just ever so slightly too light I feel. Who/what is the Diggernaut? Is it old chozo tech? Why's it hate us? I also felt like Ridley kind of came out of nowhere, and especially if this is someone's first Metroid game it'll feel like a baffling inclusion. He was also real fucking difficult, and I was almost entirely kitted out too.
Another thing about difficulty: Diggernaut and Ridley were both way harder than the Metroid Queen right? Felt a bit oddly balanced to have the main story enemy be the weakest of the three.
Also feel like it doesn't quite deliver on the quintessential Metroid finale, where you're one-shotting every idiot who dares cross your path. Even fully decked out, the latter enemies still take upwards of 3 or 4 plasma spazers to the face. It's so fun to use Hyper Beam in Super Metroid and Zero Mission. In this one enemies are always kind of in your way, and some of them even take multiple screw attacks.
But all that said, this was a fine game, and while it doesn't eclipse the giants on which shoulders it stands, I still had a very good time playing Metroid Massacre. Last question about the post-credits stinger: was that meant to be the X-parasite from Fusion? I really hope they don't go for another round of remakes after this. Let's do Samus vs. the Federation already, c'mon! Fusion clearly wasn't the end of the franchise story.