Having tried a lot of the alternatives recently on Windows 10, I (somewhat reluctantly) came to these conclusions:
1. From a usability perspective only (polished bug-free UI, good navigation and reading features, etc.), I'm not aware of better Windows desktop ereader software than...
- Kindle for PC (much as I hate saying it--I don't like Amazon's anti-open practices)
- Freda+
2. Other ones I've tried on Windows, but were not as good as these:
-Adobe Digital Editions. Blech. Reliable, can sideload local books, and create collections, but very limited features. The Kobo app is more of the same, but even more limited and locked down to Kobo books if you want to use the full feature set.
-Bibliovore (from Windows store--ok, but I liked Freda better)
-Bookviser (from Windows store--ok, but I liked Freda better)
3. Browser-based options. Have you considered this? If you're wanting to work with a single open file format like epub--then I'd suggest using Google Play Books, manage your epub files in Calibre, but sideload/upload your Calibre-managed epub files for free into your Google account. You get a beautiful, browser-based UI for reading, great reading and nav features, very good epub rendering (though: no ability to tweak CSS or style beyond very basic settings). You can also install the Android or iOS apps and sync the books on your phone/tablet. In this case, your "Windows reader" is simpler whatever browser you prefer to use. Of course, if you're not wanting to register/login to anyone's website as your OP implies, then I assume you don't have a gmail account either, and that would be required for this option.
4. Hosting your own server solution. Your OP implies you're willing to consider this. I haven't tried this option, but I'd sure consider doing it. It would give you what you want, and looks very slick, it's a complete ebook web hosting solution built by @geek101 who's a member here on Mobileread.
https://pgaskin.net/BookBrowser/
Of all these, no offense meant to Jim :-), but I find the Freda+ app while one of the 2 best AVAILABLE Windows desktop apps for ebook reading, its UI to me is rather non-standard for Windows UI design, and is often quite complex or counter-intuitive at some points, so it will take a lot of getting used to. That said, when you dig into it, it *is* really powerful and lets you really tweak the styling in a given book, as well as organize your library. It's also super flexible to let you access content from all over the place, and it's well integrated with Onedrive, which many readers are not. One thing I found particularly frustrating about Freda is its UI and handling of annotations. It's very convoluted to take a note: first you have to create a bookmark, then you edit the bookmark and you get this tiny little textbox about an inch wide to write your entire note for the selected passage. Also it's not clear in the UI and there was no documentation as to how you even get to the point to take a note--I literally had to contact Jim in the user forum to find out how to take a reading note. If that UI for annotations were improved a bit, I could probably switch to Freda and invest the effort to get used to its complex UI. But for now, I use Kindle (for Amazon books) or Google Play via browser (for all epub books). They give me 100% coverage of all the book formats I own, very rich navigation and reading features, and low friction to figure out how to use, take notes, etc.