Hi Shripal,
You dont need to create 6000 dep key. Only one dep key LINS which suits for all assets.
Dep key LINS is Depreciation=Net book value/remaining useful life. So net book value is WDV.
The major exercise you have to do is to input useful life as per companies act for all assets and changing dep key.
That too you can simply do it with Worklist and substituition if you have grouped correctly as per asset class. First create substituition vide T code OA02 and create a worklist vide AR01 and release the worklist vide AR31.
Sample screenshots are attached.
If this Substituition and worklist method not suits, you can also give useful life vide LSMW or some customised BDC.
Hope it solves.
Regards,
G.Sethuraman