there are some graded sightreading books about, though not very many – perhaps they have different steps of things that are meant to help you improve in a more structured way, rather than just lots of examples like you would get just playing loads of random music? (I don’t know if the books do this; I’m still at a low level!). Even if you don’t want to learn to do it specifically because of exams/grades, the books might still give a structure progression. What grade/board are you working on? Not that the skill would be very different, but I think I have seen somewhere something that gives a kind of syllabus on what is increasingly expected at each grade, which then might give you the ‘bits’ that you need to focus on (i.e., what patterns they expect you to recognise first, etc).
There is a series of books for the piano by an Australian company called How to Blitz Sightreading, that focuses on showing you the patterns of various chords and teaching recognition in different inversions etc., teaching rhythm reading exercises separately, and so on. I liked the look of it (pretty sure a full preview is available online) and wondered if it would be adaptable for harp somehow, but a lot of the examples were too specific with piano fingering, I think. But you might be able to find something similar with harp, or perhaps you would be better able to adapt it than I was. It had exercises also on being able to transpose the music, which might fit well with your folk background and playing by ear, because the emphasis was on recognising the shapes and feel of the chords, but then being able to play them in other positions.
I would like a harp version that included three and four note chords with the range often found in harp music (like over a tenth), and also more on chord patterns written as arpeggios/triads – you know like repeated ripply patterns that I can never quite read in time. I can process chord shapes better as solid chords (from piano) than I can when they are written out as semiquavers, for example – it’s just a visual thing, but I find it harder to recognise (beyond simple major chords), especially when you get the same pattern written over several octaves (on different staves, crossing the staves, etc – I always have to look far too closely to verify that it is the same pattern). A similar book to this that taught those patterns in a graduated way would be terrific.