This might not be the first to break, but it will eventually, and if corrected NOW will make things much smoother:

Your documentation of processes and procedures is NOT adequate.

'everyone used to know about are getting lost' - oh good thing your documentation makes this clear, because it is obviously their fault.

'New hires take forever to ramp up' - oh good thing you have complete ramp up plan and documentation, because it is obviously their fault.

'building on different assumptions' - oh good, your old assumptions are clearly documented, because it is obviously their fault.

So their is your self focus, please do good, and document well.