SaaS for B2B works well with this, you partition by organization. Each organization has their own DB. Most organizations are typically small (no more than 100 users), and it greatly simplifies things. However, it's still problematic with large organizations (200k-300k is where it starts to fall apart).
Create a separate DWH storage where all necessary data from all databases is pulled and processed/aggregated in the background (via change data capture) and saved in a format that is more suitable for analytical queries. The downside is that it requires additional preparation and some upfront design.
>Similarly, admin tools for customer support etc easily become very hard to build.
Our CRM understands the concept of organizations/tenants, so when a customer files a ticket, we know their organization, and from there, for a tech support person, accessing "coolcompany.mysaas.com" is no different from accessing "mysaas.com". In my opinion, it makes tech support easier, because you have far less data/fewer logs to analyze/sift through (only a specific organization's DB) when investigating a problem.
Nile (https://www.thenile.dev/) is trying to address this use case with a fully isolated PG databases. Though, I don't know how they handle scaling/sharding.