I'm doing a re-watch of Warehouse 13, but it's not free on Amazon anymore. IMO it's legit worth the $ if you do that (or the piracy if you risk that) though, because even when it's like
OH NO
it will NOT leave you fucked over and wishing you were dead. Episodic with serial elements rather than straight-up serial, lovable characters, adventure/comedy/drama, Weird Shit.
What IS up on Prime right now: Andromeda (yes the Kevin Sorbo SF series from 2000), all of New Doctor Who, Batman: The Animated Series (yes the early 90s one), more Star Trek than you can shake a stick at, Endeavour (period detective show that's a prequel to an old detective show, and it's actually lovely, I watched it with my mom). If you want to twitch for reasons totally unrelated to downers, they actually stuck goddamn Young Indiana Jones up there.
NETFLIX:
Altered Carbon is, somehow, as goddamn good as the books, and is WEIRD but I wouldn't call it a downer.
The Worst Witch is fucking adorable, and Galavant is dumb but funny. Arrow, Supergirl, and Flash are all up there, and if you're in the mood for soap opera superheroes, they're good go-tos for it, without all the kick-the-puppy-then-skullfuck-it that seems to be popular lately.