I actually haven't tried the Boox reader app at all! Tachiyomi is more of a downloader+reader, you can add multiple source plugins and it can search for and download (scrape) books from various sources.

It does have a few handy features for smaller screens though, such as the ability to crop borders, so you may not need to zoom into the content, plus there are various fit/zoom/crop options which you can play around with. So, at least for the comics I read (and my screen size/eyes) + Tachiyomi I don't need to zoom around at all, so the form factor is very convenient.