I do the same thing. I’ve tried Kavita and Audiobookshelf and ended up just keeping the books on a network share and then accessing them through Calibre. I am sideloading to a Kindle though.
It just stores them to the folder you choose as a vault for your notes. I have seen people put their vaults on a USB stick which they encrypt for security.
No web version of Obsidian as far as I know. Have you tried SimpleNote?
Have you tried Remote Save plugin?
I use it to sync from a webdav on my NAS at home to work computer if I ever need it. It also syncs from services like OneDrive, Dropbox, S3 etc.
There are other versions of similar syncing.
2nd vote for Obsidian.
I've moved from OneNote and Evernote about two years ago to Obsidian. I tried out (and still do look at) all the note-keeping apps and Obsidian beats hands down. For me, the major determiner was that it saves to plain text files that I can just transfer into any future app easily. The other aspect is that plug-ins enable you to tailor how Obsidian functions to your own working processes.
I've found keeping Obsidian in sync over iCloud pretty good as long as you keep the number of plug-ins on phone and iPad limited.
Just looked at Sleek (which I hadn’t heard of before) and looks pretty good. Thanks. I’ve been using the Obsidian plugin which has been fine up until now.