In Sculpt mode there is an option to set the Radius Unit to Scene for consistent brush sizes, but for whatever reason that wasn’t added to Texture Paint mode. I do agree it would be useful!
Orthographic view can maybe help reduce distortion sometimes when painting, depending on the angle, but I don’t think it makes a difference when it comes to the zoom level and brush size. I would agree with Omar and Kent that orthographic view is generally not good for sculpting, but I don’t see a problem with painting details that way if you find it helpful.
