Ditto ProV1, pretty much verbatim.
i too have a 10" shower shot that easily screws in to the slot my handheld shower attachment in my walk in shower, i have a pic in my images, you can see it here https://en.zity.biz/gallery/image/193313
i discovered my anus at age 7, so i have a lifetime of experience inserting stuff in there. To me it's a more receptive organ than it is an exit. I too use mine as a clean out tool prior to bottoming (gay total bottom here), and i have never used lube. Technically, just plain water is an irritant to that part of the body, but then, so is stool. That part of the body has goblet cells that secret mucous as natural lube. i find douching stimulates more production of my bodies own natural lube and don't really have an issue with friction.
Another trick i use is starting the the process without actually inserting the nozzle. Just putting the tip of the nozzle against the outside of the anus will result in water going in to the rectum, it's a stream of focused water coming out the tip on my nozzle, not side holes. After the first rinse, my goes in without issue or need of lube.
For those feeling particularly nerdy, here's a whole series on natural "ass juice."
https://www.pulse-clinic.com/ass-juice-part-2-how-to-stimulate-the-release-of-as-juice