Your diagram should be readable at a glance:
Layout: Clients on top or left. Databases at bottom or right. Data flows in one direction.
Labels: Every box needs a label. "App Server" not just a rectangle. "PostgreSQL" not just "DB."
Arrows: Show direction of data flow. Label with the type of communication (HTTP, gRPC, async).
Grouping: Use dotted lines to group related services. Show what's in the same datacenter vs different regions.
Don't cram everything into one diagram. Draw the high-level view first. Add detail in separate diagrams during deep dives.