Calibre has been a great tool when I wrote a book, years ago, to convert it for online stores. Only issue was that Amazon refused Calibre's mobi format and I had to make some corrections using Sigil (the other indispensable tool in this category!) The interface could be clearer and more Mac like, but it has far more functions than I needed, so I guess, it's not easy to stuff them in. ;-) The choice to make it 10.14 is surprisingly more radical than what even Apple usually does, but indeed you can still use v3.48, available on their website.