Ok, one thing you might be able to try is to take the lens off and take a picture of the white wall again. If the spot is on the sensor it should be in exactly the same place. You might need to change a camera setting before it will let you do that though.
One thing that surprised me when I found out about it was to detach the lens and shine a torch up through it. It’s probably got a lot more dust in than you might expect, and it doesn’t affect the image. Likewise I’ve had mud on the larger end and not even noticed. That and having had some really dirty sensors in the past makes me think it’s probably that.
I use swabs and fluid from Visible Dust and VSGO, you can normally buy them together in kits, just make sure you get the correct size for your sensor.
Don’t over moisten the swabs, just a couple of drops is sufficient, and don’t push down too hard on the sensor. Plenty of videos on YouTube I’d imagine.
You might want to give your sensor a manual clean with a the appropriate swab and fluid before anything as drastic as disassembling the lens. The further from the sensor, the less visible marks are, which is what makes me think this is dirt on your sensor I’ve got lots of lenses with dust in and it makes no difference.
I’ll admit I’m slightly surprised, I’d have bet money on that not being the case! Live and learn :)
It could be dirt on the rear element of the lens. It would be worth getting yourself a blower and other equipment to clean your lens with.