Mostly the fingertip, with occasional addition of the nail for emphasis (by angling the fingertip very slightly).
My nails are thin and weak. I keep them quite short so they don’t have the chance to break or split. It took me a long time to work out strumming with the index finger. Once I got it, I was pleased with the dynamic range and speed vs using the thumb.
I use mainly index finger and not like it is shown by instructors just in a way that works for me. Usually on down strum the nail has contact with strings and for up strum the finger tip. For some strums like triplets I also include the thumb where the nail hits on up strum and tip going down.
I strum with my index finger, mostly the nail. I keep the nails grown out on my right hand just for this, ok, and I’m a woman and I like to have long-ish nails. Nail erosion is not usually a problem for me, but once in awhile I break a nail doing non-uke things and I really hate having stubby to no nail then. I have bought a couple of fingerpicks, a normal one and an Alaska Pik, to experiment with the next time this happens, but as of yet haven’t had to use them, so can’t report on effectiveness.
I just got a gel manicure for the first time a couple of weeks ago and it really adds a lot of thickness/stiffness to the nail. Probably prevents nail erosion and breakage to some extent. I don’t know if something like that would be an option for you?
I don't use the index finger or middle finger but reserve them for picking notes. I use my ring finger on downstrokes and the thumb on the upstrokes. But fingers are fingers. When strumming I use the surface of the nail and not the tip of the nail. The same would apply if I used the index finger. If I used the edge of the nail like a plectrum it would freak me out because I work with my hands and have had my fingernails ripped off on wood and metal. And fingernail strumming would give me that general feeling of having my nails snagged on something.