I've researched this more than I care to admit. Some platform categories in which I would not trust a journal of private thoughts.

• Google docs - Analyzing your data is their business model. Most companies offering "free" cloud storage have a similar business model.
• Evernote, Notion, Airtable - They say they don’t analyze or sell your info. You would have no way of knowing.
• Bear, iOS Notes, anything that syncs via iCloud - Same as above. Apple says your data is encrypted. Of course it is, that is standard practice. What matters is who has the encryption keys. Apple does.
• "Standard Notes", Day One, other apps claiming to offer the gold standard: End-To-End encryption. - They make the claim, sometimes even verified by third parties. Some are open source so in theory you could inspect the code and see for yourself. Even if their code does what they claim, what stops them from changing it tomorrow?

Paper is looking better and better...

• Yes, your kids or partner could read your diary. But they won’t sell it to 3rd parties.

The safest place for a digital journal is offline.

• Unlike the for-profit software listed above, ordinary disk encryption is not proprietary. It's the closest thing we have to "do it yourself" (you’re building your own safe, lock and keys). FileVault (on Mac) or LUKS on Linux, for example, encrypts disks in a way that only the person with the keys can unlock them.
• The safest place for those keys is also offline! Don't store your passwords in google docs or any of the software listed above! From my research, an app like KeePassX is great for this.
• If you want the benefits of backing up to the cloud, be sure your data is encrypted first. I like rclone and restic for cloud backups.
• Rule of thumb: if it magically syncs from your phone to your computer, it's probably not E2E.