I've used a setup similar to Wayne's, with great success. I use an old Nikon 55mm macro lens (can get cheap on fleabay), a Nikon PK13 extension tube (27.5mm extension), A different slide holder gizmo I happened to find, and enough misc extensions (old filter rings) between the front of the lens and the slide holder to get the right magnification (35mm slide to DX sensor). The total distance from the front of the lens' filter ring to the slide is about 85mm.
A relative of mine uses a 50mm AF prime lens, with a 25mm AF extension. Works well too.
I use this on a D5100, but any DX camera would work as well. For light source I got the best results with direct sunshine hitting the frosted cover of the slide holder. I use manual exposure, around 1/4 to 1/30 sec at f/16 of f/11 at ISO 200. Perhaps f/16 is close to having diffraction issues so f/11 may be better but I don't see a difference. I use a high f-number to try and get some depth of field, since the old slides are often not flat.
If the slide has high contrast (common with Kodachrome) I find that using the camera's HDR helps. (The D5100 has HDR, the D3300 does not.) Assign HDR to the Fn button for easy access.
I use the full camera resolution (16 megapixels in the case of the D5100), but usually later in post-processing resize to half-rez because my old 35mm slides are rarely sharp enough to justify more than 2-4 megapixels. Maybe yours are better.
I also crop, and use Smart Curves and the "Wire Worm" plug-ins - the latter mostly to remove dust that always finds its way to the slide, despite my trying to clean each one (with rubber air blower, and sometimes a soft brush) before I put it in the holder. (Those plug-ins are for Photoshop but I use them in Irfanview.)
Alas those "thousands of slides" are taking me years to go through. But it's fun to revive some of the old photos. And send digital copies to family.