the job of Netflix is to provide a product its customers want. i don't think OP is in a tiny minority of people who want an option to disable autoplay; it's a desire i've heard expressed quite often. hell, fucking YouTube, a service that exists to play ads and that plays more as you watch more videos, has an option to disable autoplay
Netflix gets away without doing this because it doesn't immediately lose them customers, and implementing it wouldn't really gain them any, but i am always quite at people who declare that it's therefore good. i need to figure out some tech service that's growth positive as a function to sacrificing its users' children to Moloch, just to watch the HN crowd cry "well, it's not the service's job to /not/ sacrifice your children to Moloch, and you can put them in an anti-Moloch safe room, so I personally don't see why they shouldn't keep sacrificing to Moloch the children of those that don't bother to go the extra mile to avoid it"
Most sleep timers works in hours, maybe half hour increment. Looking up the length of a show, estimating when Netflix will interrupt end-credits, is completely unreasonable workaround.
In your situation I'd use the sleep timer setting on the TV itself to automatically turn it off after a given time. It shouldn't be too hard to align it to match the end of the show.