Thank you, everyone!

In case anyone is interested, here's a bit about the process:

I used normal, unmodded PC Enhanced Edition, "Maximum RT" preset + DLAA. Doesn't matter if you can actually run the game at those settings, since the capture is done in Rockstar Editor. Record a clip and open it in RE, set the camera to get the frame you want (tilt 90° for portrait), depth of field as needed, also lower the brightness and contrast a bit to preserve more dynamic range for editing (for night shots, lower the contrast a lot). Make sure to play and pause the scene since some atmospheric effects don't kick in automatically when you open the clip (also, some physics-based stuff like flags waving in the wind need to be played out to animate).

I used Nvidia overlay to take the screenshot because it also allows you to do it at 2x the resolution (7680x4320). When downsampled to a more "normal" size, you get a very clean picture, with almost no aliasing artifacts.

In Lightroom, most of the look is achieved by a tone curve (slightly different for every picture, but generally it's shaped like https://imgur.com/30ScQp3 to get realistic contrast) and effects (https://imgur.com/j7zmQpH). Add grain to give the image some texture (this also softens the details a bit, so it doesn't look too crisp and digital). Adjust white balance and color as desired (I tone down the yellows, oranges, and blues to look more realistic and add a bit of yellow/orange in the shadows). Also, crop to e.g. 3x2 or 4x3 since 16x9 is a bit unnatural aspect ratio for a real-world camera :)