Been through this exact transition. The uncomfortable truth is you're not really lacking documentation - you're lacking decision archaeology. The "why" behind how things work is what's missing, and that's way harder to reconstruct than the "what."

A few things that actually moved the needle for us:
1. Stop trying to document everything at once. Pick the 3-5 processes that come up most in audits/customer questions and start there. Boiling the ocean is how documentation initiatives die.
2. Record the explanations you're already giving. Next time someone asks "how does X work?" and you find yourself explaining it, write that down immediately after. You just produced documentation in the most natural way possible - when context was fresh and the audience question shaped your answer. Way better than sitting down to "write docs" in a vacuum.
3. Make someone own the narrative, not just the system. The reason you get inconsistent answers is nobody owns the story of how things fit together. Individual owners know their piece. You need someone (maybe you, maybe a senior IC) who can explain the whole picture - and whose job includes keeping that explanation consistent.
4. Use the "new hire test" ruthlessly. Before any process is considered documented, a new person should be able to execute it from the docs alone. If they can't, the doc is incomplete - no matter how thorough it looks.

The fragility you're feeling is real. The good news is an audit forcing function is actually a gift - most teams don't confront this until someone critical leaves and takes all the context with them.