Designing complex or customisable platforms with good UI?

We are currently building out a SaaS product that will have numerous plug-ins, client specific customisations, etc. We will be catering to both users that are using the product in a largely "vanilla" fashion, as well as very industry specific use cases.

As we are early days, we are not feeling the bloat of extensive settings windows, complex navigations to find toggles, etc yet. Are there any good guides or best practices to follow to make even complex products easy to use for end users? I'm conscious of products like Salesforce or SAP that are so large they require teams with explicit knowledge to manage, or times where Windows has attempted to simplify its UI and managed to irritate power users.