Abinoone wrote:
...I want to buy better lenses that I can use now on the D5600 but that also will work well on the D850 later...
Doing that you are basically shooting yourself in the foot!
The 24-70mm you mention is a fine "walk around" zoom on a full frame camera, covers a nice range for many things, landscapes included.
But it's not a good choice on an an APS-C camera like a D5600. Sure, it will work fine... but it isn't particularly wide when used on a DX camera. And "wide" is often wanted for landscape shots. A 24-70mm f/2.8 is also pretty large and heavy... not to mention expensive!
I'd recommend you get a Nikon AF-P 10-20mm VR DX lens. True, it won't be useful on a full frame camera (if and when you ever get one). But it will be a far better choice right now on your current camera. It's also quite affordable, at around $300 brand new. Even so, it's quite capable. Do some research... it will probably surprise you!
The Nikon-made alternatives.... Nikkor 10-24mm and 12-24mm are both massively over-priced.... and not really any more capable. They also don't have VR image stabilization, which the cheaper lens has.
The 10-20mm VR also is more compact and lighter weight. Don't let it's f/4.5-5.6 variable aperture concern you. For landscape work you're very likely going to be stopping down to f/8 or f/11 anyway.
There are also Sigma 10-20mm f/3.5 (big, heavy), Sigma 8-16mm (the widest of the wide), Tamron 10-24mm VC USD, Tokina 11-20mm f/2.8 DX, Tokina 12-24mm f/4 and a few others... but they tend to be a little to a lot more expensive than the little Nikkor 10-20mm. There's a Sigma 12-24mm "Art" that's full frame capable, but it's big, heavy and 5X as expensive.
Do yourself a favor... get a DX lens now to use on your DX camera. Later - if and when you get an FX camera - you can sell off the DX lens and replace it with something a lot bigger, heavier and much more expensive. The AF-P 10-20mm VR DX lens will work fine on your D5600, where it will give you roughly the equivalent of a 16-35mm lens on full frame.
https://www.bhphotovideo.com/c/compare/Nikon_AF-S_NIKKOR_24-70mm_f_2.8E_ED_VR_Lens_vs_Nikon_AF-S_NIKKOR_16-35mm_f_4G_ED_VR_Lens_vs_Nikon_AF-P_DX_NIKKOR_10-20mm_f_4.5-5.6G_VR_Lens_vs_Nikon_AF-S_DX_NIKKOR_10-24mm_f_3.5-4.5G_ED_Lens/BHitems/1175033-REG_675838-USA_1341603-REG_619515-REGP.S. Considering what you shoot... I'd recommend you also get an AF-S 18-140mm VR DX as a general purpose, walk-around lens. It can replace both your 18-55mm and 70-300mm "kit" lenses. Granted, 140mm isn't 300mm.... But you mention having a 200-500mm Nikkor and that would pair up well with an 18-140mm. The "gap" between them isn't all that critical.
In other words, three Nikkor lenses: AF-P 10-20mm VR DX, AF-S 18-140mm VR DX and AF-S 200-500mm VR FX. Especially for the latter, you may want to put some of the money you've saved toward a good, solid tripod with a ballhead and a gimbal adapter.