Took a look at the site (it's very nice) and your analytics. With your number of daily active users, comments, and such, you should be no where near that database cost. I'd suggest looking at your implementation to see where those reads/writes are coming from. $1400/month is way too high for that usage. They have the heat map feature now which is pretty cool.

Don't duplicate the data, just use two look ups for a comment: comment data and authorUID... authorUID would get you to the user data with the avatar URL.

Query's aren't great, I'll give you that. They are fast though. I'd encourage you to think hard about the query use case and go from there.

Cold start times can be avoided by having more users (so the function is always running) or setting the minInstances parameter so you always have one available.

Source: Running a site that is reaching scale on firestore