I don’t want to make a game, I just want to render some 3D models in some windows on Mac and Windows, with platform-native controls in other windows. Is that possible? It seems like Unreal Engine wants to be my whole world.
Yes, You can. UE4 has the different bridges with different DCC tools. Datasmith, livelink and etc. Connecting Maya to Unreal with Live Link | GDC 2018 | Unreal Engine - YouTube
And You can turn on “Path Tracing” mode for realistic non-realtime rendering Path Tracer | Unreal Engine Documentation