Hi, actually the feature seems to be Chrome exclusive… But maybe I can help you anyway.
The requirements are (which you can find here):
Before you turn on offline access
You must be connected to the internet.
You must use the Google Chrome browser.
Don’t use private browsing.
Install and turn on Google Docs Offline Chrome extension.
Make sure you have enough available space on your device to save your files.
Maybe Chrome is just the prerequisite because it’s about Chrome extension. Whether Google just wants to promote itself here, or “missed it”, Brave is Chromium-based, so you can install extensions from the Chrome Store.
So try everything exactly as it says in the How-To, only with Brave. You can find the extension here.
The How-To (if you don’t know it):
If that doesn’t work, you could switch between Chrome and Brave for Google Docs.
Translated with www.DeepL.com/Translator