There are other ways you can handle DDos, Rate limiting and OAuth token validation, I would suggest you take a look at this other question were there is an explanation on your options to secure Firebase Functions.
Another resource you might want to check is the Firebase documentation, especifically here whare they suggest to use Express.js middleware to deal with DDoS and securing your functions.
Finally, you can use NGINX as a sort of reverse proxy if you are more familiar with this, the only extra problems would really be that you would add an extra layer that you would need to manage instead.
Hope you find this useful!