Similar but OSS: https://fsnot.es/ Happily using it for a few months now.

Polar has absolutely nothing in common with Zettelkasten. Zettelkasten is a method of creating and storing notes, optimised for creating connections between topics and for creative thinking. The creator of this method done that on indexed cards. It's hard to explain this in few words, this book is doing this better than anything I saw online https://takesmartnotes.com/ . You can also try this article if you don't want to pay http://takingnotenow.blogspot.com/2007/12/luhmanns-zettelkas...