For port architectures, trust snapshot.debian.org
The debian-ports archive keyring does not last enough time for the snapshot archive to remain valid for the usual 2-year period between releases:
Oct 29 09:57:14 in-target: W: GPG error: http://snapshot.debian.org/archive/debian-ports/20210812T100000Z sid InRelease: The following signatures were invalid: EXPKEYSIG 5A88D659DCB811BB Debian Ports Archive Automatic Signing Key (2021) ftpmaster@ports-master.debian.org