Vet Ranch with Doctor Matt - I can't say enough about this channel. Dr. Matt is one of the most sincere, nicest people I've ever come across on the internet. His other channel is a gun-enthusiast channel which is fantastic, but even if you're not into guns don't let that deter you. His main channel follows his work as a vet, much of which is rescue work saving abandoned and abused dogs. I'd say it's one of the best channels on youtube. Again it's not really a rabbit hole anymore as he has over 2m follows (but that's not really huge be youtube standards either).
I love his editing too, he'll post the same animal's story shot over several weeks or months in the same succinct video so you don't have to go searching around for updates.
Here's just a small snippet of his work
https://www.youtube.com/watch?v=H-3Azfa72Zw
https://www.youtube.com/watch?v=_wfv3ZPR7w0