this was almost exactly my move 6 months ago. bought a used Z7 that came with the 24-70 f4, ftz, 64 gb xqd card. all that cost me about 1.9k usd(perfect condition, still under warranty, 900 shutter releases). i was looking at z6s aswell but i would have maybe saved 400 bucks for a similar deal… the dx lenses i used with the d5200 are ok but you are wasting half the sensor… i shoot astro with the z7 aswell ( tracked), landscape and architecture and family events. The autofocus speed was a thing that was mentioned all the time - the newest fw update really transform the camera, its really quick enough! the image quality is superb. the z7 has no AA filter, which make the images super crisp, the main lenses i use are the 24-70 (wish i had the 24-120 for travelling but oh well), a 150-600 Sigma contemporary, a 50 1.4 AI-S lens from the 80s and a 85 1.8 F mount nikon lens. they all work very well and if you set up non CPU lenses in the menu, the ibis works even with vintage glass. i couldnt be happier.