I’ve heard Calibre Web has a better UI and experience. Can anyone weigh in on whether that is true? I’m mostly looking for a better way to manage my book library on KOReader (Kindle) than just putting things into 3 directories (reading, to read, read) and using syncthing to copy them to the equivalent folders on my device.

I would check out Open WebUI which can be self-hosted via docker etc and configured with any OpenAI compatible endpoint so you can use a service like OpenRouter to run nearly any LLM remotely. Most of the open weights ones like Qwen 3 or Kimi K2 Thinking are great and cost pennies per inquiry and can be configured with Zero Data Retention (ZDR) so your data is not recorded. You could also use something like Ollama to run local LLMs if you want even more privacy and have the hardware (typically a modern Nvidia GPU with at least 16-24 GB of VRAM).