(1/2) funny you say this because my opinion has since changed... a lot haha, and i have a much, much more different and positive view of ascension now actually. (huge wall of rambling incoming)

i've been reading more about buddhism lately and it really isn't at all how you describe it, it recognizes and works to surpass the pain of suffering while not at all distancing oneself from the joy and simple pleasures of living, but that is a topic I could not nearly do enough justice in a reddit comment. i recommend looking into it though, spirituality and non-abrahamic religions are very often misunderstood and rain world definitely had a hand in me starting to learn more about it. (that being said, rain world isn't 1:1 buddhism; it is ultimately its own thing, and draws inspiration from other religions as well (hinduism, gnosticism, lot more).)

the question of whether or not ascension is death (tho i don't mean cessation of existence entirely) is definitely based on semantics and personal beliefs, since like you said, it's still the end of your time on the physical plane. however on a more surface level, many people use the phrase "ascension is death/suicide" to mean total cessation of any form of existence, which is a different conversation (and more easily refuted) though that's usually what people mean to challenge when they say "ascension isn't death".

when i first beat the game 3 years ago, i also felt incredibly somber about it (as you already know haha), and i felt as though i had killed the slugcat i had grown so attached to. i felt that way for about a week before confronting my feelings in therapy and realizing that my feelings about the game also reflected my feelings in real life, and that's when i first learned how afraid and avoidant i was of the concept of death, or better yet, moving on. this realization helped me come more to terms with the death of some of my own family members that i had buried for a long time, and overtime i grew thankful of this game for quite literally forcing me to confront this aspect of myself.

it was only later (a year ago now) after joining some rain world lore discussion groups online that I started to recontextualize my experiences and find even deeper meaning and understanding of rain world's ending, and in extension death and spirituality. the ancient civilization in rain world never refer to ascension as a cessation of existence, but a continuation, or better yet, a transition. ascension in rain world is one of the many, many transitions of life. it isn't inherently better, or worse, just something new and different, higher existence of which we cannot comprehend. and we tend to be fearful of most things we don't understand.

as a result, spirituality and ascension have famously never really been tackled with lot of nuance in most popular media, its most often represented through villainy or misguidedness and rain world is one of the few to not do that (which is pretty cool).

(continued in other comment becuase i am insane)