I do like this idea of integrating SRS into doom-scrolling streams! I've wanted to prototype something similar, but built completely around your SRS cards.
For email, I think you're right it would need to be frictionless. There are some new email technologies that let you embed dynamic interactions in your email - we might play with that at some point.
So this is probably hard to do as a plugin tbh, but I'd love to explore eventually as a feature.
This is what we've built RemNote for! You can add your sources (PDFs, websites, YouTube, etc.) and create cards directly from them. Either with AI (click on a sentence => get an AI card), or by highlighting a sentence and writing your own card.
It's not at the OS level, but I think a focused standalone all-in-one experience is actually better. We've explored a chrome extension here that does something similar to what you suggest, but it somehow hasn't proven useful in practice. I always keep coming back to just doing everything directly in the tool (upload PDF, take notes, make cards), as it helps me focus deeply on what I'm learning.
RemNote founder here - been building RemNote for the past 8 years (4 in private for myself, 4 in public), and we're not going anywhere! SRS is the future.
For email, I think you're right it would need to be frictionless. There are some new email technologies that let you embed dynamic interactions in your email - we might play with that at some point.
So this is probably hard to do as a plugin tbh, but I'd love to explore eventually as a feature.