Plugin architecture for SaaS extensibility: lessons learned?

Our team is working on building a plugin architecture for our B2B SaaS platform, similar to how Shopify apps extend their core product. We want third-party developers to be able to add custom functionality securely and scalably. We're considering WebAssembly for sandboxing, using webhooks for event notification, and providing a REST API for data access. What are some hard-learned lessons or best practices for building a robust and developer-friendly plugin system? How do you balance extensibility with security and performance? What kind of developer experience (DX) considerations were critical for successful adoption by external developers? Any specific patterns for managing plugin deployments and upgrades?