Another tool to look at might be Dendron[1]. It's a VS Code plugin with similar functionality to Obsidian (according to the docs, it actually evolved from being a standalone app like Obsidian into the plugin form). My understanding is the client/plugin are FOSS, and the developer plans to implement some (optional) server-side functionality to fund ongoing development.
I wrote my own, if it ever gets to a point of not being terrible I will open source it as a not-as-pretty FOSS alternative.
I think the second brain notion is very powerful though, regardless of platform.