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.