This is such a sharp breakdown — you've articulated exactly where the AI "agent" promise hits reality.
We deal with a very similar problem on the engineering side, but with different sources: ERP systems, payment gateways, LDAP, digital signatures, legacy databases with 6k-line stored procedures. The "Reach" problem is identical.
A few things we've learned (not AI-specific, but integration-specific):
On historical context — no, we haven't solved it without significant engineering. The pattern that works: event sourcing + a unified read model. But that requires each source to emit events, which most SaaS CRMs and ad platforms don't. So you end up with poll-based ingestion + idempotent processing, which is ugly but works.
On multi-tenant for agencies — we've done both. Client-owned infra is cleaner for compliance but slower to iterate. Managing it yourself gets faster but you own the PII risk. Hybrid pattern: thin per-tenant ingestion layer (client's VPC) + centralized control plane.
Is it solvable at scale? I think yes, but not with off-the-shelf connectors. The hard part isn't connecting — it's handling schema drift, rate limits, partial failures, and backfills. Once you build that substrate once, it becomes reusable. Most teams just rebuild it per project.
You're not alone hitting this ceiling. The gap between "AI as copilot" and "AI as operator" is 90% data infrastructure, 10% prompting.
What's your current stack for ingestion? Airflow? Dagster? Something custom?