If you listen to the audio, you can hear the engine kind of catch, and the rpms slowly start to build. Even when the engine is spinning at double the starter speed, it still doesn't run right.
Investigating the starter might fix the random "no turn over" problem, but it won't fix the poor starting when the starter does run. That is barking up totally the wrong tree.
This engine is either missing fuel, compression
, air, or heat from the glow plugs.
My understanding is that glow plugs can have eroded tips, but still test OK for resistance. Try replacing them. Also, have you checked that they are getting power?
This engine starts like it does not have working glow plugs.