> between 1.5x and 2.0x is the sweet spot for me, and almost no apps provide that.
Agreed, this is super frustrating and is some evidence that the market isn't that interested. (Why would they pay for advanced compression if no one bothers with trivial stuff like finer gradation?)
Fwiw, on Android I use Audible, PocketCasts, and Audipo for audiobooks, podcasts, and general audio, respectively. They are all serviceable and have 0.1x or finer granularity.
I've taken the website down but you can still view it on archive.org: http://web.archive.org/web/20160329053206/http://www.lectrap...
I still use Swift for finer gradation. It also worked for video files, which was fantastic (mine never did that). Swift is 32-bit and will stop working soon. I haven't found a replacement yet.
(On iOS, at least, the 1.25/1.5/2.0 thing is because that's what the OS provides for very little effort. Finer controls require use of a supported-but-undocumented API or AudioUnits.)