Hi guy's. I have been shooting with my Nikon D300 for a lot of years now. What would be a good upgrade for me,
In my opinion, the best reason to upgrade is to fix a problem or fill a hole you have with your current gear. The question to ask is, "How is my current camera limiting me?"
Example: With my Sony A6000 the biggest limitation is the speed of my (kit) lenses. The obvious upgrade for me woudl be an 18-55/2.8. But I'm too cheap to spend $1,400 on the lens (or even $550 on the Sigma alternative) so I make do with what I have. (That money buys a LOT of film cameras, which is what I prefer.)
I suppose you could say "Wait, he shoots a Sony a6000? Now THERE is a guy NOT worth listening to." But I think it kind of makes my point -- my Sony is six+ years old, and I love it and cannot come up with a good reason to replace it*. I can't bring myself to upgrade for the sake of upgrading. I love the camera's size and form factor, I'm happy with how my photos come out, and while I would love another stop or two out of the lens, that's not too much of a problem to work around. So I keep the Sony.
* I have found one good reason: Going full-frame would allow me to use my film lenses at their "native" focal lengths, and that'd certainly be useful... but the only camera that would match my Sony's form factor is the A7C, and until those come down in price, I will have to stick with the Sony.
I would say a good upgrade would be the camera that eliminates the thing in your D300 that is holding you back. If there's nothing in your D300 that is holding you back, I don't know that an upgrade will make you all that happy - unless you love getting new stuff, and there's nothing wrong with that.
FWIW I helped a friend shoot a wedding using his Nikon D850. I was so excited to be using such a top-end camera. The effing thing damn near gave me a hernia. Turns out my friend was shooting with his "lesser" D810 because it was the camera he knew better. He spend big $$ on the D850 when it first came out and I don't know that he's found any advantage to it.
Anyway -- Hope my grumpy old opinion helps.
Aaron