What lenses do you have?
Do you want to shoot more in low light than you have with the D90?
The D500 is today's equivalent to the D300. Very fast and accurate AF, big buffer for fast-paced shooting and ergonomics that lets the user do most things without going into the menus.
If you want to shoot birds in flight, the D500 is the best tool for that job.
The modern equivalent to the D90 is the D7500.
FX will give you lower noise levels and greater dynamic range than the DX sensors, but you will need FX lenses to use the advantage as DX lenses don't fill the larger frame. You can use DX lenses, but in crop mode. This will lower the resolution to a lower level than the D90's unless you go for the D850.
If you have any old screw-drive lenses, forget about going mirrorless yet as they won't AF on the FTZ adapter as there is no AF motor.