I want to third Kagi. I switched to Kagi solely over a year ago and have not looked back. The search is so much better than Google or Bing. Another bonus is that you can pay via Lightning through their OpenNode integration.

As far as customization goes, I can have it give me only links directly to PDFs, only search certain academic journals that I select, only search Reddit, etc, etc. It prioritizes sites that have less advertising or tracking in the results too.

The creator of Kagi has a project called TinyGem that is a repository for links and content discovery. Might help you organize some of your thoughts too.

Obsidian is another tool that requires some work on your part, but then would be a tool to search through everything you've mentioned. You can even hook up open source LLMs running locally to help you sort through your Obsidian database.