The D850 (and Z7) come (as Telecaster noted) relatively close to DX pixel density, but nothing else in Nikon's line does... On the Canon side, the 5Ds/5DsR used in crop mode are close to Canon APS-C cameras, and for Sony, the A7rII and A7rIII are close to Sony's APS-C line. Any other full-frame camera will have many fewer pixels when cropped than a dedicated APS-C camera. The few 36 MP cameras (D800, D800e, D810, original A7r and Pentax K1 series) crop to about 15 MP, the 30 MP Canon 5D mkIV crops to about 12-13 MP, and everything else crops to around 10 MP.
Of the FF cameras with enough resolution to serve as viable crop cameras (assuming you aren't willing to live with 10 MP images), there is one in particular that stands out. The D850 is a very quick camera that speeds up even more in crop mode, and it has an AF system that works very well with long lenses. If you have a D850, the only real reasons to also own a crop body are if the last few pixels matter to you (D850 crops to 19 MP, D7500 is 24 MP - but note that the D7500 is not as capable a body as the D850 in terms of focus or durability - the very capable D500 is almost exactly the same resolution as a cropped D850) OR if you want a second body anyway, can't afford a second D850 (again, or a Z7) and shoot enough wildlife that you'd rather have the crop performance of a D500 than something like a D750.
If your FF body is a Z7 or one of the Sonys, you might well use it in crop mode. A Z7 with a long lens on the FTZ will outperform any Nikon crop body except the D500. The rest of the lineup are consumer-grade cameras that won't match the performance of a Z7, but a D500 will significantly exceed it. Z7 and D500 are a very interesting pair, because each has strengths where the other is weaker. The Sonys, especially the A7rIII, are very good performers, and Sony doesn't really have a tempting high-end crop body to pair with it (although there are a lot of rumors).
The Canons are really not viable crop choices for other than occasional use. The 5Ds series are slower than you'd want for wildlife or other
crop-friendly subjects, and Canon has a couple of quick APS-C bodies that are much better choices. Of course, if you happen to be holding a 5DsR with a 600mm lens on it, and a great bird photo presents itself, it'll work just fine. It's just not the camera of choice if that's what you're after most of the time.