Best lens for portrait photography on nikon d750?

So i'm planning on buying a new camera that is nikon d750 but i don't know which lens should i go for right now i have a tamron 24-70 2.8 g on my nikon d7200 and i'm not satisfied with the pic quality so i'm going for a nikon d 750 with a nikkor 85mm 1.8g and my question is that should i sell my tamron lens along with my nikon d7200 or should i atleast keep the lens and use it with the new camera (d750) or should i sell it and buy a new lens?

If you're not happy with it sell it off, honestly. The new camera won't make that much difference with that lens and you are biased anyway. Start with a clean sheet.

If you are NOT satisfied with the photo quality using the 24-70 mm, buying a new lens will NOT help

Sure the Nikkor 85 mm or 105 mm lenses are great portrait lenses, but if you are not able to get high-quality images using your current lens, you need to look into your skill set, using what you have.

Do you have a full understanding of the fundamentals of photography, lighting and composition? Unless you have those fundamentals well in hand, NO camera or lens will make your images any better than they are now

You can use that lens and your camera to shoot decent portraits already. If you can't, buying a better lens and camera won't help you.

24-70 f2.8 on an FX sensor is a good general use lens and should be retained if you do not require image stabilisation. The 85mm would be a good choice for portraits and will rule out all factors other than user-inexperience and lack of supplementary accessories.

https://www.cameralabs.com/Nikon_vs_Tamron_24-70mm_comparison/2

70-200 f/2.8 is by far the workhorse lens used professional portrait photographers.
You could also go with the 24-70 f/2.8 if you're constantly needing a wider focal length because you're shooting groups in tight interiors. Ultimately you'll own both.