I’ve been reading books on my iphones for years, using the native Books app, along with Kindle and eBookMobi; most of the books I have are in the books app, but I have quite a few in the others, mostly due to DRM. I guess in total I’ve got around 700 ebooks.
I’ve never had an issue reading on a phone screen, it’s just black text on a white background, except when the light dims, in which case it’s white on black.
Same with my pad, it’s just a bigger book-sized page is all. The advantage of the phone is it’s always with me, and it’s a handy pocket size; the bigger screen size of my 11 Pro Max does make reading so much easier than the screen of my iP4S, though… 😁