I'm right before (what I assume) is the final boss. I've really enjoyed it though I don't think the "stealth" sections were implemented quite as well as they could have been based on some other 2D stealth games.
Overall though, yeah - it's wonderful to see a 2D Metroid being released in 2021. The combat is really fun and challenging - I haven't really kept count, but I am thinking this might take the prize for most bosses in a Metroid game.
I also like how they changed the expected order of getting certain items from earlier games. It's not quite a Breath of The Wild level reinvention, but it's certainly a neat approach.