To answer your question:
Does Debian already provide signature on .deb files (that is, provide a manifest of their hashes and and sign each)?
Yes it does. If you look at https://ftp.debian.org/debian/dists/trixie/InRelease it's a PGP-signed file containing a list of files and their hashes. Each of those files (eg https://ftp.debian.org/debian/dists/trixie/main/binary-amd64...) then contains a list of .deb files along with their shasums. In other words, a Debian repo is a set of deb files, metadata files with their hashes, index files with hashes of the metadata files, and PGP signatures for the indexes, so the whole chain can be verified.
This means that anyone can set up a deb mirror by (essentially, there's some extra steps) copying that entire structure and the integrity is guaranteed because only the upstream admins can sign the metadata.