I read The Plague dogs some 15 years ago, and it certainly has played a role in my awakening to the plight of animals in the anthropocentric world order. I don't think I'd want to touch the film. In the book, the author Richard Adams cuts in from the first person point of view of the dogs to explicitly state that he can't bear having such a bleak and horrible ending, so is going to change it. As for a comparison to Watership Down, the latter is about human behavior and society as much as about animals, but The Plague Dogs is very much an anti-vivisection book.
I'm not much of a Watership Down film fan, it's far too short, too much of an overview of the story. I haven't seen the series, and have only heard middling things about it, but I think that's the right format. Actually, a Fannie and Alexander/Scenes from a Marriage-length series/oversized film would be great. I did like the animation and voice acting, though, so maybe Plague Dogs would work out. I don't find Watership Down depressing, though. It's got its rough spots, but it ends up OK. The Plague Dogs, with the original ending maintained by the author, would probably wreck me.
Re: Cult of Disney, I know I'll always be reckoning with that. It's less "cult" to me than it is "machine", though. If I ignored all extratextuals like the buying up of competition, the more "machine" part, I'd still be left with the very heteronormative white culture it reflected and perhaps helped to maintain, and figuring out if any of the tweaks to the formula and even the critiques of present Disney creators on the former works matter. I have no problem going back and watching their films and saying when they get things right, but there are surely some (Snow White, The Lion King, and The Little Mermaid come to mind immediately) that I know are very problematic. But yes, if I'm ever to do an animation marathon, Disney will have to be a major part, I can't just ignore their existence. It does draw on my bigger inner-conflict which is my love of certain segments of pop culture (E.T., Star Wars, Saga/Popular Image Comics Titles, Stranger Things, classic Pixar, LOTR, Song of Ice and Fire (as opposed to the Game of Thrones series, which only interested me for a short period of time), Harry Potter, Nintendo, PS4, yeah, a lot) vs. my love of subverting norms and pushing against the current cultural paradigm that causes too much suffering and sadness. But that's a good internal conflict worth having, and never being able to perfectly balance or understand it is 100% OK. Makes for good contemplation and conversation.