Extremely curious what kinds of data your app would produce such that it outgrows the memory available on a single client device. I have about 2 person-decades' worth of writing, art, and coding that occupies less than 10 GB (and could probably be made smaller).
I'm not talking about media content, rather database records. PKM apps host in-process relational queries, the records must fit in working memory and query engine must traverse the working set indexes to return answers "instantly"