I would say given the small amount of research you have provided us with, I suggest you do not start with such expensive equipment. These full-frame cameras are the tools of the professionals. Hardly the choice for your first camera. If this was your second camera and not your first camera, I would recommend this stay with the brand So you can use the same lenses with the new camera (although lenses designed specifically for APS-C do not work fullscreen).
My advice would be to make a list of lenses and other devices (Canon, Nikon, flashes, tripod cameras, etc.) and choose the one that looks like a better package. Many Nikon sensors are manufactured by Sony (Exmor sensors). Although they are less loud, they are worse for astrophotography. So if you want to use the Sony Exmor sensor, it may not be the best choice. In low light conditions Exmor may have an advantage over Canon sensors.
APS-C accomplishes most of what offers full-screen at a lower price.
My suggestion is a camera with APS-C sensor, a cheap normal lens (in Canon that would probably be the EF-S 24mm F / 2.8), a 50mm lens (Canon EF 50mm F / 1.8 ) and the kit zoom supplied with the camera. Then you can purchase a flash, a remote shutter (if necessary for the flash, some flashes are fired by another flash), a tripod, a tripod for the flash, spare batteries, a camera bag, etc. If you are still inferior to your budget Only with this extra equipment would I consider the possibility of a full screen.
I would say, 1500 USD, you can buy a good starter kit with APS-C sensor and all other required equipment. With a full frame, you could even double the investment (full-frame lenses tend to be more expensive, at least for Canon, with the exception of 50mm lenses).
In general, the photographer, the lens, the light and the tripod determine what kind of photo you will end up with. A 1.6-fold increase in the horizontal and vertical dimensions of the sensor (from APS-C to full screen) is not so important.
To edit: And if you're really sure that new frames are your thing, then you should opt for mirrorless frames like the Canon EOS RP. The lower weight overcomes some of the disadvantages of the frame, namely the weight.