This is an excellent pattern; do as much as you can in the database.

External Spanner provides changes streams. Internal spanner is different, mostly because of the extreme scaling requirements in some cases (and a healthy dose of "because it already works" mixed with "arbitrary change streams are scary").

Internal Spanner allows any transaction to write queue entries, where queues are (more or less) tables with some special time awareness. You can schedule delivery. Entries get pushed from queues to a handler which can also do writes to the DB within the dequeue transaction. And all of the same scaling is there.