Depends on your threat model and what you're intending to guard against. You're running an unsupported version of macOS so we'll address that first, if you don't mind losing 32-bit app support I would consider upgrading to Catalina at least. 10.14 Mojave stopped receiving security updates in 2021.

If you're worried about someone physically getting your laptop I would enable FileVault in System Preferences with a strong password (you can enable it for the boot drive and also removable drives like USB sticks), and make sure you shut the laptop down whenever you're leaving it somewhere.

TextEdit can work for writing simple documents but LibreOffice is a good option too, but the macOS port isn't very good (lots of bugs and it runs slow af for me but YMMV)

Just kidding... mostly.

Realistically, your diary is safe enough on your Mac. (By the way: you say "I am certain I need to install a Linux distro or a BSD," but technically, MacOS already is a BSD.) IMO the main threat vector you have to worry about would be Apple changing something to start harvesting your data itself, either for its own purposes (e.g. AI training) or legal compliance with Orwellian bullshit. That risk is a lot lower than the certainty it is with Windows, but it's still proprietary software so it can't be zero.

If you're really that paranoid about it, any device without an Internet connection would do just fine, including something that isn't on the Internet because it's too old to be compatible with it (hence the typewriter, but a very old computer running DOS would do fine, too.) Otherwise, if you want a maximally-paranoid but still modern system, look into something like Tails or Qubes OS on a PC, or Graphene OS on a mobile device.

If you’re worried about it, zip the file up in a password protected archive. No need to change oses or anything like that.

You can set a password to your Libre Office file, that way it gets encrypted at rest. You can also use alternative software like Joplin that offers encryption (you have to set it up, but its as easy as toggling it in settings and inputting your new password). Your system might be an issue though, its quite old and might be insecure, but because its such an old machine it might be easy to put Linux on it. Maybe you have someone around you who can help you with it? Linux is not a necessity. Its more about having up-to-date system. But on your current laptop, Linux might be the only way.

get an offline device like a writerdeck

You could perhaps have TailsOS on a USB drive (with persistent storage enabled on the drive: for your writings), and after use remove the drive from the device and keep it well hidden. That seems like the most attainable method as far as I'm aware.

There is no way to avoid your writing getting copied if its in digital form because even on a single device it must make copies of it in order to function. So your choices are a) encrypt it or b) don't digitize it.

I also suggest Joplin + password setup. It will cover all your writing needs with a good categorization. You can even connect to internet and backup it on a remote location, since backup file will be encrypted. MacOS is fine, you don't need Linux.

I don't know what tools you would use for this on MacOS, but you could use some firewall implementation to block internet access for user and system apps that your are concerned about. If suspect that unwanted connections are being made to specific hosts (IP addresses or URLs) and you know them, you could block access to those specific hosts with a hosts file or, again, with your firewall.

To summarize the above:

1. read up on firewalls for MacOS
2. read up on hosts files/DNS based blocking for MacOS

You could also simply disconnect from the internet while typing, then saving and encrypting the saved file, deleting the plain text file and only then reconnecting to the internet.

Lastly, don't put effort into pursuing privacy blindly. Formulate your threat model. What do you need to protect? What do you want to protect? From whom? Why? Without this, you'll burn yourself out.

You should be fine wiþ þat stack. You don't need Linux; I've not read about Apple scanning laptops, yet, and you aren't using a web-based editor.

If you're concerned about someone getting to your files, easiest is probably getting a folder encryption tool like VeraCrypt (þere may be better options for OSX - I don't know þe space). Unlock your folder when you write and lock it when you're done, and it's probably good enough for most þreat models which don't involve state actors.