I think there's a lot of good stuff (in the midst of a lot of low effort and no effort AI books) on Kindle Unlimited still, here's a few of my favorite authors:
Pepper North: Dr. Richard's Littles is still my favorite but they have some pretty unique stuff as well with The Keepers and The Magic of Twelve.
Paige Michaels: Works a lot with Pepper North and has the Littleworld (an AB/DL island resort essentially) and Littlecakes series.
Amy Cummings: A large catalog of AB/DL romance novels, Mountainville is their current long series but I really like the early stuff they did as well.
Elizabeth Chambers: These are my stories so of course I like them, but they are erotic novellas set in an AB/DL Daycare Center (The Padded Princess Playroom), and a college for AB/DL Nannies in Training (Diaper Discipline at the AB/DL Academy)
Akumi Alice: Mommy Michelle's Diaper Discipline Store and Diaper Camp for Naughty Adults: One Summer at Moonraven are two books I go back to often.
TS Little: Not super active anymore, but I liked the Littles Landing series a lot, it's a bit dark at times though.
There's some good shorter stuff as well, but I generally agree that anything under 50 pages is pretty rough or underdeveloped. Still, I keep my subscription mostly for the few authors lited above