Short answer
For a small marketing-ops team handling ~500 leads/day, start with Zapier to move fast and validate the workflow; migrate to direct API integrations (or a simple queuing+worker system) only if you hit cost, reliability, or complexity limits.

Why: tradeoffs in one paragraph
Zapier wins for speed, visibility, and no-dev maintenance: you can wire triggers, run lookups/dedupe, call enrichment services, and post to Slack in hours. But it has per-action costs, limited control over retry/backoff, and can become brittle or expensive as volume, concurrency, and complex stateful dedupe needs grow. Direct APIs require dev time up front but give you better performance, deterministic error handling, batching, and lower marginal cost at scale.

Recommendation (practical):
- Phase 1 (now, 0–3 months): Implement in Zapier. Use Zapier Storage or a lookup table for dedupe, call enrichment connectors (Clearbit, etc.) or HTTP requests to your enrichment API, and post structured Slack alerts. Instrument each zap with logging and an error Slack channel.
- Phase 2 (metrics-driven): If you see rising action costs (>~$1000/mo), duplicate rate >1–2%, latency or retry issues, or compliance requirements, plan a migration to a lightweight API-based pipeline with queueing and workers.

Decision criteria — when to stay on Zapier vs build APIs
- Stay on Zapier if: low engineering bandwidth, predictable low volume (~ $1k/mo or duplicate rate >2%).
6) Migration plan: design API endpoints with idempotency, use a queue (SQS/Rabbit/Kafka) and small worker service for dedupe/enrichment, batch enrichment calls, and add observability (logs, metrics, traces).

Notes on budget/skill: Zapier is cheapest in time and staffing; APIs are cheaper per lead at scale but require dev resources and ops. If you want, start the Zapier build and keep a simple repo/spec for the eventual API migration.

CTA
If you want a template Zap flow or a short spec for the API-based pipeline (queue + worker) I can produce one — and I can link a starter Zapier setup to speed your MVP.

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