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:
- read up on firewalls for MacOS
- 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.