It might still make sense to keep the canonical metadata in xattrs so that tools can interoperate on it and build a search index from it when you actually need

It might still make sense to keep the canonical metadata in xattrs so that tools can interoperate on it and build a search index from it when you actually need to do search. On a NVMe drive rebuilding the search index shouldn't be too costly, even on hundred thousands of files. I think there were also some ioctl additions for btrfs that let you sequentially walk the filesystem by extents or inodes instead of by directory tree, which makes indexing a lot easier.