IMHO, there is no general "sweet spot."
There however is a "sweet spot" for YOU.
What I mean by this is
1 - If the "general sweet spot is a D7200" but you can't afford it, than it is of no use to you. So your budget defines the boundaries of the sweet spot.
2 - If the lens is not what you need/want, you will end up replacing it, at further cost to you.
So, the sweet spot has to be what you can afford, and what you need.
Example1, I use the Nikon 18-140 lens. It has worked out better than I expected. The 18-140 has met 80+% of the shooting needs, and significantly eliminated lens changes. Right now, I only change lenses when I need a FASTER lens for indoor sports, or a LONGER lens.
Example2, but for casual home/family use, the 18-140 may be too bulky and expensive, and the 18-55 might fit that need better.
In fact, I have been thinking of getting either the D3400 or D5600 with a 18-55, just for use as a 'grab camera,' for family parties and such. Easier to use than the bulkier and heavier D7200, and better than my P&S camera.
I can tell you from personal experience, if the gear is a hassle to use, you won't use it as much as you should. This is why phone cameras are used so much. You have it with you all the time, and they are easy to use.
If I just want an upgrade from a P&S, the D3400 + 18-55 might fit the need.
As you can see YOUR use of the camera drives which camera and lens meets that need best.
Try this.
Draw a matrix, with columns for D3400, D5600, D7500.
Then list the features of each camera, so that you can compare what features each has.
Then REALLY think about these features and if they are of value to you.
Example, the D7000 has the ability to autofocus the older mechanical autofocus lenses.
If you do not have any of these lenses, then that feature is of no value to you.
However if you do have these lenses, then that feature is of value to you. It was for me.
Then list the needs or things you are interested in.
Then under each camera, note how or if it meets those needs.
Overbuying is like insurance, so you are not caught short of functionality later. But that comes at a $$$ cost. If you never end up using that functionality, you wasted $$$. But no one can predict the future, so this is a personal decision that has to be made.