How to take backup of multiple DB in a specific point of time
You can try using the --snapshot method (https://paquier.xyz/postgresql-2/postgres-9-5-feature-highlight-pg-dump-snapshots/), but it might be a bit more complicated than you'd like.

My personal recommendation is to create a cluster-wide backup with pg_basebackup (or a third-party app that basically does the same thing, like barman, pg_backrest, etc.)