Got to say this setting off my inner analyst. Now I will say its design is a stark departure from most Star Destroyers that we see and it's blunted end and rounded more softer edges does not fully fit into the mindset of the Tarkin Doctrine we see in Rebels: emphasising fear and intimidation.



However I feel this is its main advantage and allows it to stand out against the more overt a danger shaped designs. Also it feels like it would fill a nice niche roll of light escort but that's it as it does not appear to manuverable due to its brick like design.



As for when it was made I feel this variant was constructed in the post Yavin and after the death of Tarkin. As his death would allow for a shift away from the Capital ship obsession the Empire had. If anything it looks like something Thrawn would build: its simple unembellished and while not flexible would be simpler to build and crew than a ISD and by your description packs above its weight.