This was my mindset for my leopard gecko I just put down recently. I spoke with my herp vet and he said the same thing. If it's just closure I'm looking for he'll do it, but I should really just mend myself and then put that money and effort towards a new baby to take care of. Petry was old so I'm just going with the idea that he lived a long life with his previous owners and then me and it was just his time to go.
OP go for the necropsy if that's what you want but it honestly sounds like this snake just had it's own issues that weren't visible. Doesn't sound like it was anything you did. I hope you find some peace of mind in whatever form works for you and I'm very sorry for your loss. It's never easy to lose a pet.