Ah, looking at the source, it's probably scaling the rectangle by the
contentScaleFactor and so producing a texture 2x or 3x as big, and so exceeding texture size limits on lower end devices.
The fix is to provide a scale when you initialise the RenderTexture, e.g.
_renderTexture = new RenderTexture(1600, 900, true, /* scale = */ 1);
That might though create a texture with a lower resolution than the screen. If this is a problem then you will have to find a workaround, such as splitting the texure into two or more pieces if it is too big.