My case is different from yours as I focus on street photography, but I recently got my first camera, an a6400, and I’m extremely happy with it. Took some ~30k photos so far.

I wouldn’t start with full frame unless you have a lot of money to spare. The lenses are much, much more expensive, and if it’s not something that sticks you’ll be throwing a lot of money away.

At this point in time, people like you and me are still figuring out what kind of photography (and thus equipment and lenses) we like, so I think it’s better to experiment with less expensive gear before committing.

Whatever you choose, please make sure you figure out the ergonomics.
I have an a6100 and I find it big and heavy when I hike with it.
I recently started carrying it on a capture clip on my belt and it made it significantly better.
Don’t forget to check the lenses too. Weight, price, size.

Expect to spend as much or more on the lens than the body. A quality lens is more important than the body. Personally I use a Fuji XT-4 and 16-55 f2.8, 10-24 f4. Here’s some example pics: https://www.Flickr.com/photos/tcgoetz

Find a camera that you like to use.

35mm isn’t the be-all and end-all like some like to say. I really wish this mentality would go away.

Your budget needs to include some lenses; the other comment about lenses is pretty much spot on. No point spending so much on the camera when you can’t do what you want because you’re now stuck with limited lenses. You didn’t say what you want to shoot, so that could cover very broad subjects, from insects to birds to landscapes (a full high end kit to set that up at the 35mm end can easily be 4-6kg).

As sensor tech hasn’t advanced that much in the last 10 years or so, it’s better to get an older higher-end camera than a budget new camera; the controls and UI tend to be better, which will be better for you to learn photography with. Budget cameras tend to cheap out on controls, so having to menu dive to make basic setting changes will get old very quick.

I’m not saying I recommend this one, but I’ve been using a Cannon SL2 since 2017. It was their basic entry level APS-C camera at the time. And I wouldn’t immediately discount crop sensors; they have a place in the photography world. I would actually recommend starting with a crop sensor due to the better price and your more beginner status. The biggest benefit of full frame over crop is better low light performance. They have bigger pixels so they by default take in more light. I’ve been using the crop sensor for 6 years now as a hobbyist and I still don’t feel like I’ve moved beyond what the SL2 can offer me.

I focus on nature photography as well and have taken several awesome shots with it (check my post history if you’re curious). The benefit of a crop sensor is it “zooms in” the image for you compared to a full frame. It’s not actually zooming in; it’s just the crop sensor compacted the pixels into a smaller area, so less of the lens is used. But this allows you to “zoom in” further than a full frame while using the exact same lens. This could be useful for getting a shot of a subject that’s really far away.

I agree with the other guy about ergonomics. A reason I went with the SL2 is because it’s so small. It’s one of the smallest DSLR’s around and is literally just a few grams under a pound. That makes it significantly easier and more comfortable to hold for long periods of time. I have a big chungus 150-600mm lens for getting close ups of birds from far away. That alone is 4 lbs. Pair that with my camera’s body and I’m holding around 5 lbs of equipment. That gets heavy quick. If I had an even heavier body, it’d be even more cumbersome. I know a monopod is best for that situation, but sometimes you need to free hand it.

Another thing to look for, ergonomically, is how the body fits in your hand. I remember an old review for an older Sony camera. It was one of their smaller bodies and the reviewer mentioned that the handle’s bump was too close to the lens mount. So they could never get a good grip on it (I’m sure it’s been fixed since then). If you rock bigger hands, you may want to go for a slightly bigger body for comfort.

Another thing to consider is lens support. A big reason I went with Canon/SL2 is because of their wide lens library. Canon have just about everything I could need or want. And if they don’t, I can always look to third parties. My crop sensor is also able to use Canon’s full frame lenses. I don’t think Sony or Nikon do that.

My last bit of consideration is to focus on mirrorless cameras instead of DSLR (I didn’t look at the cameras, this is just general advice). It’s looking like that will be the future of interchangeable lens cameras. Canon made an SL3 in 2019 to replace my model, but they haven’t updated it since in 4 years. And no rumors of an SL4, either. It would stand to reason Nikon is moving away from DSLR, too.