Hello Mike, and welcome. I was on the dark side myself, in film. I shot Canon's F1n, and had planned on using Canon digital cameras also, but turned out I like the feel, controls, and quality of Nikon.
For low light in concerts, a D750 would be hard to beat, although the shutter isn't exactly quiet, if that's a concern. The D750 does most things very well, it's gained the reputation of ''The Goldie-Locks'' camera, as it's just right for so many things. I just bought one myself for low light photography and am favorably impressed.
With the new Z6 II out there's a glut of hardly broken in Z6 cameras on the used market. I looked at the Z5 myself and consider it one of Nikon's best kept secrets with many features of the Z6 and Z7 at a much more affordable price. The compact Z50 looks very good also and can be had as a kit for around the cost of a new D750 alone.
I'm sure Peter will be along to give you better guidance than I can...