I just stumbled upon this app that is in closed beta. From the blog on their website, it seems the philosophy behind the app is similar to thebrain. Anyway, have a look ...

https://blog.brainio.com/our-story/what-is-brainio/

Looks promising. I’ll bite.

Thanks. Worth trialing the beta. If they get to adding the "import your Kindle highlights" feature, that would be very useful.

Thanks!

I had a feeling of deja vu while going through their website; then I remembered Octopus Note https://www.octopusnote.com/ discussed here https://www.outlinersoftware.com/topics/viewt/6445/ which is the same concept, as far as I understand. It has been in Beta since three years; I wonder what happened to it.