Horses for courses! A good post.Why not just use your iPhone?
This is a serious question. There might be good reasons. But write them down. Perhaps you are a scuba diver or you want to shoot pictures of birds in flight or you are shooting sports and the iPhone will not react fast enough to te controls. I'm sure you have reasons. Make a list.
Then when selecting a camera make sure it addresses your list
I bet some things you wrote in your post will not show up on your list. For example "interchange legacy DSLR lenses" is unimportant if you do not already own serval SLR lenses
You don't need many "features". Just control the focus, focal length, ISO , aperture and shutter speed. And image stabilization is very nice to have.
The most important thing is your location and the time when you trip the shutter.
So many beginners think photography is all about the equipment. They say "If only I had a better camera..." I'm sorry but t is like a painter saying "if only I could afford a better brush." The iPhone allows you to select the composition and you can choose to lighting and it allows you to be at the right place at the right time. It has a lot going for it.
Try this. Take two dozen shots EVERY day. Give yourself different assignments like
1) You are a tourist from far away visiting your home town and you want tot shot those back home what this place is like
2) Find and shoot contrasting colors
3) Shoot street portraits (ask before you shoot and get their cooperation)
4) find and shoot repeating patterns.
5) any other photo 101 class assignments you can think up.
After you have two weeks of work find the best dozen photos.
Now ask how better equipment would have made your new portfolio of a dozen shots better. maybe there are real good answers like
1) I could be not missed some split-second timing
2) I could have framed the subject better
3) I could have reduced camera motion blur.
Another assignment: Go to the library ad look at books of images by well-known photographers and figure out which person's work and style you like. Now spend a week or so shooting images attempting to emulate that stype. Perhaps even copying some images. Make notes on how your current equipment prevents you from doing this
I still shoot a 15-year-old Nikon D300s I only use a few lenses, some very old. No one has ever told me "These images would look so much better if you bought a more up-to-date camera." I never hear that. You can buy a used D300s for $150.
If you want VALUE, look at the used market. Buy something for $200, shoot 4,000 frames and then sell the camera for the same price you paid. After 4,000 frames you will know for sure what you want.
For years, no decades photography students were told to buy a simple SLR and a single 50mm lens. All of their assignments were to be done with this setup. The idea was that they needed to learn about photography, not about gear collecting. But now I say "Start with your cell phone camera and only buy more equipment if you can say exactly what problem you are having and how the new gear will address that problem.
I despair of our staff taking clinical pictures on their work phones. They are just not up to the job indoors. They have a variety of cameras from D300 up all with macro lenses and external flash, but it is a faff and it is easier to use their phone.
Outside running/walking in the hills I use the iPhone all the time and it is perfect, unless I am photographing a race or on a bit of a landscape project. Photographing insects on the bench the iPhone is great!
What the OP has not stated is what, where and when and this makes it difficult to be precise with an answer. However if the purchase is not a whim a new mirror camera, even a cheaper kit one is likely future proof and will allow the OP to learn most of the technical tricks. A D300 is not compatible with quite a few lenses now and if going 2nd hand I would say based on a huge market go for something more modern.
Composition is both personal and the downfall of many, relying on Photoshop to crop and correct etc. Cameras are only a tool and some of my best pictures from a birding trip were taken with a D2H - the light was amazing and I had time and the tiny sensor was an irrelevance.