I too like best the page number display of Apple Books for iOS. Numbers are shown in the left margin. In cases where there there is a blank page before a new chapter or section, publishers will tend to put that blank page # either at end of that section at the end of the last XHTML element, or sometimes together along with the next page number in the next header. Showing in margins seems like it'd work best overall. When UX controls (back, TOC, search, font/display change, bookmark) are hidden, page number is shown in the footer.
I like the idea of a Sigil standalone reader. Last time I tried them, the majority on macOS were clunky, had a slow UX, the UX was not particularly attractive, and so forth. I use either ADE or the calibre reader; calibre being not particularly feature rich. It would be terrific if a reader had some features such as cache (so re-opening a book was nearly instant) and a control such as sidebar to show search results one could navigate (like most PDF readers). Vertical scroll mode (like Books for iOS) or how PDFs can display on many apps would be nice though perhaps difficult to implement. A particularly nice feature of Apple Books is that it parses the entire book into pages, and thus the scroll bar when scrolling shows an overlay with target page number as well as TOC section. Such is terrific and would be if possible wonderful.
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