Yup, Long Term Learning is similar to SM-2, and we use an e-factor of 2.5
We opted for only two answer options mainly to keep the study experience as simple as possible. It simplifies the implementation a bit as well, because we can treat the “Know” button as equivalent to a q=4, meaning the e-factor is constant.
From a learning perspective, I don’t think user-set scores are absolutely necessary. The key part of SRS is the exponentially increasing study delays. Allowing the user to fine tune the back-off rate for each term can improve learning efficiency a bit, but you’re getting the main benefit either way.
I don’t think of long term study as a shift exactly. We want all students to learn as efficiently as possible, whether they’re studying over a short term, long term, or somewhere in between. And we want to encourage even short-term studiers to space their study out more because that’s going to lead to better results. Having a good long-term study solution is just a part of that goal.