I think: Any unused RAM space can be allocated to read write cache. My own system has similar to yours.
I think buffers refers to space dedicated to storage. SATA/SAS allow NCQ command queues. For SAS, that can be 256 commands, across 8 queues, per drive - or so I believe. All that takes RAM that cannot be freed and is essential to the operation of your drives.
Buffers may also include system buffers, temporarily allocated, for the running application.
Buffers are mandatory space. Cache is optional space.
Someone please jump in if this is incorrect.
I'm running 6 Cams, so Surveillance will probably pre-allocate some buffer space for each video stream.
Free space is just space left over by an application that has terminated. The system will slowly recycle and use that space but allocating it to Cached. There will probably always be some Free space as that region can be used instantly, while Cached requires an application to wait while the system Cache application is requested to let go of some of its space. That is not instant, but quick.