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.
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.