A paper-only journal would defend against the state, but not against people you live with. A digital journal can be encrypted, but an intelligence agency could potentially gain access (like, them reading your anti-government rants that may involve violence… that sort of stuff).
So… how to defend against both threats?
(Also, I just realized, paper journals cannot really be easily backed up…)
There are no shortage of encryption software in the market, use one, any one, Veracrypt is fine, so is million other software for this.
Use a note taking app like Joplin, Obsidian, though not open source is just as fine. Sync its data folder to the encrypted vault created with encryption software of your choice, and you are good to go