To follow up on my brief comment last night - I've finished this now and have to say that overall it was really, really good!
I love the movement and control of Samus in this game, for my mind it's probably the best the character has controlled in a 2D game to date. The moveset is varied and interesting, every power and upgrade brings a tangible benefit to how you traverse the world and deal with enemies and learning to make the best use of these is really enjoyable.
I do still think that Samus's jump is a bit borked, though not to the same extent as in some earlier games. I still found myself getting occasionally frustrated when she fails to enter a spin jump for some reason (why even have two types of jump?!), or by the overly finickity timing window for when you can initiate a follow up space jump (why not just let me double jump at any point after the first jump like basically every other game ever made?!)
The graphics were far better than I was expecting based on the early trailers and I actually think it ended up being a really nice looking game with some lovely design and atmosphere. Performance was pretty solid for the most part but there was still more slowdown than I would have liked which was a bit disappointing. Particularly toward the end when I was going around mopping up items it seemed to start really struggling at times. Loading times left quite a bit to be desired as well, transitions between areas really should be quicker than what we had here. I was replaying Mario Odyssey over the weekend and that game can fling Mario between these massive open 3D worlds in a far quicker time than this game seems to manage to load which makes no sense and makes me think that they either made some bad design decisions about how the game was put together or simply didn't have enough time to properly optimise things at the end.
Speaking of things being slow - some of the UI aspects could have been better, again mainly thinking around area transitions. Why they feel the need to make the process so slow baffles me, especially as you end up doing quite a lot of zipping around the areas by the end. The sequence of stepping on the floor pad, waiting for it to activate, waiting for a menu to pop up to show you where you currently are, then highlight the transition between your current area and the next one, then zoom in to show the map of the area you are going to, and then FINALLY giving you a prompt to say "Do you want to go here?" is insane - maybe do it like that the first time but after that if I stand on the pad just ask me straight away if I want to travel, or at least make all of the UI nonsense skippable!
Bosses were almost universally excellent in this, I really enjoyed the variety they offered throughout the game and have to say that I find the final boss to be fantastic - a really nice finale that encouraged the use of lots of upgrades that the player had come to be familiar with over the course of the game
Overall I don't think this will take the crown for "best Metroid" in my book - it doesn't match up to the tone and atmosphere of Fusion or the brilliant design and world building of Prime 1 for me, but it's still a fantastic achievement and easily my favourite Switch game of 2021 so far!