I nearly bought a P900 a few years ago for the exact reason you have it, but due to other constraints (only JPEG, etc etc) I didn't pull the trigger.
As mentioned the 2,000mm you can't match at the price you mention without giving up a lot of functionality (ie, really old manual focus zoom lenses). Plus you are going to gain a lot of weight.
For instance you can find an old Manual Focus Tokina 150-500 lens for about $500 used and add that to a Nikon D5600 about $700 new and you are at $1200 for 750mm and VERY BIG and HEAVY by comparison to the P900 .. about 1250mm short of your small sensor and extra "zoom" P900 camera too.
Above $1000 .. say getting close to $3000 you have options of a Tamron 150-600 and 1.4x teleconvertor and a nice 1.5x camera, which would give you about 1300mm, still short of your 2000mm
Though I've never compared the image quality of the P900 to a cropped image from a DX camera in those scenarios.
How far away are you from the planes??
You best best to move to a DX camera is jumping in with a 70-300 VR AF-P lens. which as mentioned is far short of your 2,000mm P900.