You're right, it's not bestiality and idk why everyone thinks it's so.
1. The snake can talk and has the same mental capacity as a full grown human (she even thinks he acts like an old man)
2. The snake is clearly a god, or a creature on some higher level of some sort, seeing as how EVERY ANIMAL IN THE FOREST came to watch him hibernate.
3. He's not the size of a normal snake, even the biggest snakes on earth don't reach his size. I get it's be weird if he was normal sized but he's not.
By all accounts he's a human in a snake body. Sure I guess one can think it's weird he once was a normal snake, but he's been a not normal snake for 500 years, to the point where theres a whole festival about his existence which certainly didn't come into existence recently!
I mean it kinda depends on the definition of bestiality that you use.
The most common definition is simply a person (human) having sex, or preforming sex acts with an animal (non- human). From my really quick google search this definition is the most common and Intelligence isn't really factored into this definition. So inspite of Daija being a unnormal snake or a god it still counts as bestiality because he is, of course, a snake.
Now the meriam webster dictionary defines bestiality as "sex between a person and a
lower animal" with this definition Daija and Miyo getting down and dirty doesn't count as bestiality because Daija is in now way
lower, even though he is a snake( non- human animal).
personally i believe,while morally okay, its still bestiality simply because there isn't a better word to describe this type of relationship (human - equally intelligent non- human) and it is literally a woman having lovey dovey baby making sex with a snake then again crossbreeding donkeys and horses together doesn't have a specific name so i guess the former case really doesn't need one either.
P.S. Daija has mentioned once that although he is long lived, he really is just a snake, not a god. The scene with the animals seeing him off for the winter lends credence to him simply being an "unreliable narrator" but Daija himself claims he's just a snake.