I'm going to give you something different to think about compared to the other answers so far. You might not like it but hopefully it'll make you pause to think a bit more and could save you from wasting money. Upgrading to get better high ISO performance is worthwhile.
But to get a greater depth of field with the type of photography you favour you need to improve your technique and choose the correct lens, and you can do that by remaining with an APS-C sized sensor such as you presently have. A 35mm lens is far less than ideal for portraits; if you were using a full frame sensor it gives the approximate equivalent view of a 50mm lens. The traditional "best" portrait lens for full frame has generally been in the 105mm to 135mm range, and on what you have now that's around a 70mm to 90mm lens to get a similar angle of view and can be achieved with your 50-200mm zoom.
Portrait lenses allow you to stand further back and give a far flatter and more natural facial appearance and because you're a little further from the subject the model tends to act more naturally. They also give you greater depth of field so you don't need to stop down the aperture setting so much, but equally you need to be aware that most lenses are at their optical best from centre to edges at about f/5.6-f/8. For the very slight slight softness which suits portraits f/4 is better.
If you feel forced to shoot at the widest apertures all the time whenever you select the least noisy ISO settings (so up to about ISO400) then you need to add more light, and that can be as simple as using cheap reflectors. Upgrading to Nikon full frame FX bodies simply won't get you the extra bang for the bucks. But it will certainly be a literal pain in the neck as the bodies and lenses are all considerably heavier and far more expensive as you're already aware.
So you can achieve most of what you wish by going for the D7100/D7200 and then using your 55-200 at the 80-90mm setting and ideally at about f/5.6. But with the right lighting you won't even need to upgrade the D90 as that will allow you to use low noise ISO settings on that body where you will not see any appreciable difference with an upgrade. In short, better technique, better lighting (and the skill to manipulate the available lighting), and more intelligent use of the gear you have now completely removes the need to upgrade. That then leaves your budget free for better upgrades in future as the sensor on the D7200 and D7100 may be good, but it's still behind that used on cameras like the Pentax K5ii and later Pentax models which all have superior high ISO with low noise performance but Nikon will get there.
Money can burn a hole in your pocket particularly when you've been researching and getting ready to buy new gear, but the best research is when you look and realise it's better to wait. In the meantime a little bit of investment with portable handheld reflectors and perhaps a good tripod will deliver far more bang for the bucks. The reflectors can even be free; it's amazing what you can do with just a newspaper or white card or with aluminium cooking foil (scrunched up and then unfolded gives a less harsh reflection).
Reflected light from the Financial Times (printed on pink paper) gives a lovely warm tone to pale skin and richens the tone of dark skins.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.