Hey MQSF, welcome to the WFG community!
The layout of the site is quite nice; the minimalism of the layout makes it simple to read and easy to see what's going on. If I had to guess, I'd say your high bounce rate comes down to three things:
1) Like you noted, the site becomes all but un-navigable on mobile devices. If I hit this homepage on a mobile device (https://i.imgur.com/IPBEqAB.png?1, https://i.imgur.com/oxD0YFt.png, https://i.imgur.com/CJKsUBU.png) and I was just vaguely interested in checking the site out I don't think I'd continue.
This actually wouldn't be too hard to fix, I don't think. It looks like you've got some custom CSS in there, if you just threw in a block to make the "portfolio" div responsive instead of fixed, I think the home page would work out more like you intend on mobile devices:
Doing something similar on the serial pages, or updating the chapter titles to also link to the content, would help there.
2) The chapters are in PDF format. This is great for some people, especially those who want to read the completed work, but the majority (if not all) of of Web Fiction readers are expecting chapters as HTML pages on the site. Some readers have their browser set up to format color and font the way they like, some readers use "read it later" apps or extensions like Instapaper, some readers use article-readers to have serials read aloud to them, and having chapters in PDF makes all of those special cases break.
Between the readers who expect the story as part of the site and the readers who need the story as part of the site, I'd imagine this is contributing to the bounce rate as well.
(As a sidenote, they are absolutely beautiful PDFs. Very nice formatting work there)
3) Readers are so freaking skittish. If a reader can see that they'd have to do something to read Chapter 5 of Tantalus (let alone pay money), they're not going to start Tantalus unless they're already an active fan. If Tantalus was the first they happened to click, they're probably not going to bother checking if Exiles is similar. This is super unfair, and not the way things should be, but the sad truth is that most first time "I'm checking this thing out" readers have 10 other tabs of Web Fiction open, and the slightest reason to close yours will be enough.