Cost-effective observability for 50+ Kubernetes clusters without vendor lock-in?

We're running ~50 clusters across three cloud providers and the per-node observability pricing from the usual suspects is becoming unsustainable at scale. What's your approach to centralized metrics/logs/traces without locking into a single vendor's agent? We've evaluated OpenTelemetry Collector + Loki + Tempo but the operational overhead of maintaining the storage layer is the blocker. Curious about real-world setups that keep costs predictable while retaining query flexibility. Jurisdiction: AGNOSTIC Scale: ~50 clusters, ~2K nodes total, mixed GKE/EKS/AKS.