I use NoteSelf, which is a variant / add-on / expansion of tiddlywiki. It requires an accessible CouchDB backend hosted somewhere on the internet, but once that bit of infrastructure is setup it's almost universally accessible.

I've barely scratched the surface of tiddlywiki's features, but linking ideas together is easy, in that you can link individual "ideas" (in the form of individual articles, referred to as tiddlers) from each other, and it also supports tagging, and there are built-in scripts for iterating tiddlers with chosen tags.

No affiliation, just a very happy user.

https://noteself.org/