Hey there! I was in a similar situation what I was looking for a camera set up. I get asked a lot about my set up, and it's a pretty simple one. It's over your $1k budget, but it's also still a good value for what you get.
Body: Canon 6D Mark I
Lens: Canon 100mm w/ image stabilization
Porthole: Building an Obsession (I've used other ones, and this one is the best by far)
The Canon 6D Mark I is an older model, so it might be hard to track down. But it's pretty cheap for a full frame DSLR, and I know Vivid Aquariums is using the same camera last I saw Brandon (their main photographer and sps god reefer). I purchased mine for roughly $1k back in 2017. You might find some deals on eBay or your local marketplace for a used one. The Canon RP is the new and mirrorless version of this camera, and is a great camera for those looking to get into photography. Much lighter and smaller! This would be my choice of body if it was available to me back in '17. You can purchase a Canon lens adapter to use their EF lenses (like the 100mm). This would be great for general photography for vacations and trips, or if you want to get your feet wet into photography in general.
The Canon 100mm is a workhorse! I use it almost daily and it's great. Than of Tidal Gardens also highly recommends this lens as well, and his photos are phenomenal. Unfortunately it costs as much as the camera body, but this is the norm. Camera lenses hold their value very well and good glass will last a long time. It's a pretty long lens though, so your coral subjects will need to be about 12" away from where you're looking at.
The Building an Obsession porthole is a pleasure to use. It's lightweight and attaches to the bottom of your camera's mounting. Other portholes use plastic screws to basically pincer themselves onto your lens, which can damage your equipment overtime. You also have to make sure that you've attached all 3 screws evenly, or your porthole will be angled compared to the lens. It ranged from $50-$80 depending on which model you purchase.
Here's some examples of photos I've taken to show what the current gear I've listed can do. I've borrowed my friend's Canon EOS R, which is similar to the EOS RP, and it produced the same results but faster and more efficiently. Due note that the photos don't come out like this straight out of the camera. You will need to use a photo editing software like Lightroom to help out with White Balance, cropping, etc. Everyone has their own style of editing.
Cheers,
Darwin