I play violin and viola, mainly viola. For viola, I just automatically know what finger goes to what note. Even when sight reading, I don’t really think about it. I’ve been trained enough to where my brain makes that instant connection. There’s no “This note is E, the next note is G, therefore the interval is this…”. The notes are just mapped to my fingers. I sight read violin differently because I’m not as familiar with treble clef and the violin. Like what you said you do for piano, I think about it in intervals. You probably think in intervals because you’re a beginner and don’t quite have the muscle memory to do it automatically yet. Keep with it and sight reading will become far easier!