Selfhosted

Ebooks: I use Calibre locally and Calibre-web on the server (read-only metadata db, I overwrite with the Calibre version as tagging, etc is far easier on desktop).

You can connect Koreader to Calibre-web and until maybe a fortnight ago you could jailbreak a Kindle and use Koreader instead of the default software. Now you'll need to manually move files over, or use the email-to-Kindle option (probably a bad idea, but I expect Amazon can tell if you've side loaded pirated content anyway). Nowadays I buy from not-Amazon sources, strip any DRM and send it over.

Manga/comics/graphic novels: I use Kavita on the server and I use comictagger on desktop to fix the metadata.

I'm happy to use different set ups for the different types as they're quite different experiences and specialist tools work better.

I went through essentially the same thing a couple months ago. Tried Calibre (and Calibre server) since everyone recommended it.

Really disliked it. Calibre is great for converting ebooks, but has shit management and webserving capabilities.

I ended up with Kavita and am super happy. On the web client, both management and actual reading are a pleasure. Any phone/tablet client supporting OPDS works perfectly to read/download your manga/books from the server.

And a select few clients go a step further, supporting Kavita's API, which allows for 2-way sync (effectively, syncing reading progress between all your devices).

Android tablet phone here.

Manga - Mihon

Ebooks - ReadEra premium

I use calibre on my PC to manage ebooks, but I often download directly on my phone.

I use TTS (Text to speech, paid feature) to listen to my ebooks. Sometimes I use @Voice (free with ADs) to listen to books. It's better for that.

I would like to start managing ebooks and manga properly.

I guess my question is how is everyone using these services for their own library :)

I moved away from dedicated readers. They're nice, but I have a tablet, a phone, and a laptop. I don't need a fourth device with me.

For me, the major selling point for dedicated readers is that they buy eInk: their insane battery life and how they work very well in sunlight or otherwise brightly-lit conditions, so you can read outside.

For comics

I don't know if you're only viewing black-and-white manga

my understanding is that color eInk displays have limited contrast compared to the black-and-white ones. I think that if I were viewing anything in color, I'd probably want to use some kind of LED or LCD display.

I will occasionally read content on my Android phone with fbreader. The phone isn't really a great platform for reading books

just kind of small

but it does a good job of filling the "I'm waiting in a line and need to kill a few minutes". With an e-reader, you need something like Calibre to transfer books on and off, but with Android, I can just transfer files the way I normally would, via sftp or similar. I don't have any kind of synchronized system for managing those books spanning multiple devices.

I use an Android tablet sometimes, almost always when I want to cuddle up on a couch or just want a larger display or want to watch videos. Same kind of management/use case. I think I used fbreader to last read an epub thing. I've switched among various comics and manga-viewing software, am not particularly tied to any one. There's a family of manga-viewing software that downloads manga from websites that host it; I can't recall the most-recent one I've used, but in my limited experience, they all work vaguely the same way.

I've increasingly been just using GNU/Linux systems for more stuff, as long as space permits; I'd rather limit my Android exposure, as I'd rather be outside the Google ecosystem, and the non-Google non-Apple mobile and tablet world isn't all that extensive or mature. For laptops, higher power consumption, but also vastly larger battery, and much more capable. On desktop, it's nice to have a really large screen to read with. For comics

and I haven't been reading graphic novels or comics in some time, so I'm kind of out of date

I use mcomix. For reading epubs, I use foliate in dark mode. I have, in the past, written some scripts to convert long text files into LaTeX and from thence into pretty-formatted PDFs; I'll occasionally use those when reading long text files, as I have a bunch of prettification logic that I've built into those over the years.

I don't have any kind of system to synchronize material across devices or track reading in various things. Just hasn't really come up. If I'm reading something on two different devices, I'll just be reading two different books at the same time. Probably have some paper books and magazines that I'm working on at the same time too.

I've tried komga and kavita both, but didn't find them very useful. Nowadays i just copy the files downloaded from nyaa to my kobo, where i read them with koreader. Its possible to sideload it on kindles, but its tricky. Koreader even has OPSD support, in case you want to use kavita/komga. But for most random/weekly reading I use Kotatsu on my tablet/phone, as it has a sync feature (you can even selfhost your own sync server!). For ebooks I also just use my ereader. I have no need for a bulky management system. I hope this helped you somehow. Good luck!