Best depiction of highly advanced space combat I've read is in The Culture series. Extraordinarily brutal, measured in milliseconds and hundreds of kilometers apart at minimum, but usually more like AU apart.
There's one scene where a human(ish) woman is aboard a true warship during a near-equivalent tech engagement. The human is strapped down, armored, and pumped full of exotic nanite foams just to survive the momentum changes, and has to have the combat played back to her over minutes to understand what happened.
I'd say its fairly realistic within it's own world building, but that world building is supposed to represent a type 2 civilization in it's prime, capable of manipulating higher dimensions, so not hard scifi.