I have a Boox tablet (an older Note3) for the actual reading, I run readarr as a downloader/manager and use Ubooquity as the server. If you arent a massive nerd I'd probably suggest a kobo reader over an android reader.

I dont tend to "stream" the books from the server, because there is no point, they are tiny files, so i use the ubooquity webui to download the file to the device when needed. though even that is unnecessary as I can just vpn into the server itself and pull the files, or have them all sync automatically when on wifi since it is just an android device so i can run whatever apps I want to do that, I just use ubooquity as I used to use its web ui reader to keep in sync between multiple devices but stopped reading on my phone as I preferred the e-ink display. could also just dump them to a usb-c disk and move them manually.

I might soon replace ubooquity alltogether and just have Readarr put the files into nextcloud or something directly and have that sync with the tablet when on wifi.

The source for the titles themselves is the usual suspects, public trackers, usenet etc.

I've used calibre in the past to convert and de-drm books for a kindle I used previously, but I never actually needed any of its other features like re-formatting or editing metadata so I stopped using it as soon as I replaced the kindle with the Boox reader.