RSS is s data interchange standard. "subscriptions, progress, already-read, or any other kind of state across apps" is well out of scope of a data interchange format.
You may claim that the data interchange format is useless without those features being implemented on the app side, however it doesn't mean the data interchange format is flawed.
That said, pretty much any RSS app does what you describe. I use Feedly across many devices, and it's all synced. The other primary RSS powered app I use is Apple Podcasts which also syncs those states between all my devices.
Feeds being "public" isn't an issue. I pay for many feeds provided by creators on patreon. These are behind token based auth. I also have a few completely custom feeds that are behind http basic auth and generate on demand completely custom content.
Monetization for public feeds also doesn't seem like an issue. I get ads embedded into some feeds, and others like Ars just do snippets and I click though to the main site and see the ads there.
Why would crypto help at all here?