Photographing oil paintings with the highest standard of fidelity is time consuming.
I believe your camera and lens are sufficient for the task.
With a decent tripod and some patience it is possible to carefully center the camera.
The tricky part is the lighting. To control the lighting all the light should come from off-camera sources.
- You must avoid lighting with mixed color temperatures.
- You need to minimize glare and reflections. As Doug mentioned a polarizing filter is useful. A circular polarized filter on the lens is better than nothing. But the reflections and glare comes from a wide range of angles since an oil paint surface is three-dimensional.
- The 3D nature of the surface also compromises using 45 degree lighting angles.
- The optimum method uses cross-polarization with three linear polarizing filters. One goes on the lens. The other two go on the off-camera flashes or strobes. The lens and flash filters are set to be 90 degrees off axis. LP filter film is inexpensive so you can make you own flash filters.
- Add a small of reflected light. When the polarization is set to completely eliminate glare, all brush-stroke detail is lost. This means rotating the lens filter by very small amounts can add just enough reflection to define the brush strokes without obliterating large regions of color detail. You make several exposures with slightly different lens filter orientations.
- With strobes you need to avoid hot spots. This means you might to use wider apertures.
- If you use continuous lighting, constructing the polarizing filters is less convenient to avoid melting the filter film.
- Use a grey card to calibrate the color temperature in post-production.
- Use raw files and auto-bracket three aperture exposures by 1/3 stop.
- The X-T2 can be tethered to a computer so you can optimize images in real-time.
- For the final image rendering make sure your display is properly calibrated.
Sometimes the reproduction fidelity is not a high priority. This makes the job easier and less expensive.
An alternate method is to natural lighting. This requires patience. Outdoors you have to wait for a very cloudy day with calm winds. The light must be constant so the cloud cover needs to be thick. Now diffuse light comes from all directions and surface reflections are not an issue. You may need to flag the lens to minimize veiling flare. Some people with have access to large rooms with large windows. They use large reflectors to create even lighting with one color temperature. They use window scrims to diffuse the light to eliminate surface reflections.
1. If the strobes must be moved away from the painting the result will be a wider aperture which could reduce the lens' MTF50. I would start with base ISO (200 for the X-T2). In this case just increase ISO. Increasing ISO reduces the analog dynamic range. But in this case the DR will be well within the sensor's range with even a two-stop ISO increase. Shadow region detail decreases when ISO increases. However for art reproduction one does not selectively push shadow regions. With the X-T2 (and other pseudo-ISO invariant cameras) increasing ISO to maximize lens performance should not significantly degrade perceived image quality.