I wonder if this compresses so well is because of some of the same words being the multiple times (
previous, ids). Basically a lot of this tangle stuff. So what I would try to do is to do the same for your full log.
Another question I have is how does this interact with db2?
The log is stored encrypted. In the case of this brotli, you would have to know that the feed is compressed, so you have to decompress first of all to validate. Then whenever you get it from the database you decompress as well. Because the compression time is so large, I don't think it makes sense to store it decompressed.
One thing you can do with meta feeds is that you can attach metadata to it, so you could say that this feed is compressed with brotli and the other end, if it supports it, could then chose to replicate and handle that feed accordingly.