Reading large .mbox email files without importing to Mail?
You have to use a third-party app, on MacOS I haven't found any free ones (on Windows you have some free).
This one for example: 
https://apps.apple.com/us/app/mbox-office-viewer-migration/id829623734

All messages in an .mbox mailbox are stored as plain text so you can also open it with any text editor.