The D850 is a mega upgrade from the D80 I agree, it's like the jump from Windows Xp to Win 11.
But It's very OK camera no objections PRO level and 45.4 Mpx, awesome for Astronomy and Landscapes.
If you want to continue using DX lenses would be an inconvenient because of the crop, like Ron apunted.
If you consider another DX DSLR the best choices are D500 and in the other hand D7500 or D7200.
But I suppose that if you check the D850 is because you want a camera with HI Mpx and FF, so in this case D850 is the top of the series of this kind of cameras, and you can find it second hand.
But not confuse quality with Mpx or better results, you can check my post about D3, It's an FF flagship PRO with only 12 Mpx, the lowest Mpx in FF Digital series.
D3 second hand - Nikon FX DSLRs - NikonForums.com
Advantages : Low MB files in Raw, the sensor and the low Mpx and the good ISO ratio is ideal to use all old lens without problems, eat almost all crystals you give.
First, you must be clear the question for what kind of photos you want the camera or what lens you will use or have.
Like I said, D850 unbeatable for Astronomy and Landscapes but is quite exigent about lenses because of the 45.4 Mpx, all those details sometimes are like .....
Double edge knife, magnifies the mistakes with a sharp detail opposite to other cameras. Old FF lenses don't combo well with D850 is a fact, you will see better results with this lenses in lower Mpx cameras.
But with the new lenses and doing the strong point of D850, You will have big Raw files that you can edit and crop with software, loosing less quality than others cameras.
Usually, the users of D850 have more than one camera.
But D80 is very obsolete camera, you need an upgrade yes or yes.