Museums and galleries nearly all do not allow tripods or monopods.
YOU are the tripod so image stabilisation, in screen level and framing guides are ideal.
A wide aperture low distortion lens is best, such as the AF-S Micro-NIKKOR 60mm f/2.8G ED.
As a backup for higher paintings, that you will have to step back to shoot, the AF-S VR Micro-Nikkor 105mm f/2.8G IF-ED. But you do not want to be constantly swapping lenses. Stick with whichever suits your distance from the subject or take all the low level paintings with the closer focus lens, then go back for the higher paintings with the longer focus lens.
Lighting, reflections and other people being in your way are the biggest obstacles to getting good photos in galleries. Patience and organisation is all you can use for the later.
The lighting is usually subdued, and flashes are banned to protect the paintings, so a good wide aperture lens is essential. Learn to take photos with a steady grip and pay attention to what you see in the viewfinder. Most bad photography is due to not paying attention. Not seeing the framing, reflections, shadows and bits of people's heads or bodies intruding. So the solution? Use your eyes. See what is there not what you hope or wish was there.
If you say you are not a great photographer, maybe you should use a smartphone with good camera such as Apple's iPhones which do a remarkable job of keeping the colours and detail, although not the best for low light.
I found Google's Pixel phone cameras are remarkably good for the price and they have levelling and squaring guidance on screen. They are light and you nearly always have them available when you want to photograph anything on whim. Get a good quality phone camera (Check the DXO site for ratings/reviews) with image stabailsation and good low light capabilities.
SLRs are heavy and clunky and should not be your go to unless you are at least semi-professional.
