Dust on sensors is a “fact of life” with DSLR’s
First thing to understand is if you can see the dust spots in the optical viewfinder, it is not on the sensor.…
The sensor is behind the shutter.and can be seen only when the shutter is open
The spots that you are seeing when you look thru the viewfinder with the lens off are on the ground glass viewing screen and will not show up in photos. The ground glass view screen is above the mirror 5 is the bottom surface of the ground glass viewing screen and is easy to clean. Dust can also be trapped on the top surface and that is major requiring the body to be disassembled. :) It may bug the heck out of you, but it will not show up in the photographs. So most of us live with it.
4 is the location of the se