Upsizing in 10% increments is far too conservative and presents its own set of issues. A 16x24 image from your original is a modest change and within the realm of a single step. Your original inquiry about 24x36 is a different matter.
I have recently sold a 24x36 canvas print from a D300 image and had to resubmit the image due to noise so I am very sure of the issues and potential solutions.
You can't look at them the same and see the difference. Downsizing to a common view eliminates the artifacts. You need to look at the image zoomed in to the actual size of the planned print.
4200x7200 at 300 ppi is 14x24 inches. You'll actually need 4800 x 7200 for 16x24. For 24x36 you'll ideally want 7200 x 10,800 at 300 PPI but you could probably live with down to 250 PPI or 6000 x 9000 pixels.
On my enlargement, the problem was in the shadows. But anyplace you have highly processed can show artifacts. If you have an even exposure that did not require heavy adjustments, you may have less of an issue. I find a light touch with noise reduction is often required for skies and smooth areas.
I use BayPhoto for my large canvas prints. They do a great job.
If you are printing on canvas, be sure you leave enough extra for the edges. You may need to add canvas and mirror edges or stretch edges depending on your image.