In the early days, when I had this problem, I tried all sorts of things that did not work. Eventually I rigged up a gadget of my own that involved two anal catheters (left over from my incontinent days) connected to the shower hose.
The first one had the tip removed so that the water flowed straight out of the end in a fast flowing jet. This cleaned the upper regions of the channel.
The second one, the tip was left on, which meant that the water came out from the sides of the tip and with a twisting movement would clean around the sides.
I stood in the bath to do this as there was really no other way as the volume of water would have been too much for anywher else. Also, it was somewhat satistfying to see all the muck that came out and float down the bath.
This proceedure was used for about six months before the lumps of hardened mucus stopped appearing; the mucus became clear and no longer smelled dreadful. I have not had to repeat this process in years so I presume that this is a newbie's problem.
Sorry to hear you have these problems as I can certainly empathise with how you feel.
PLEASE NOTE: I do not recommend trying the same procedure as it has got to carry serious risks when poking an anal catheter up a newly operated on anus. The only reason I tried it that I was already used to anal irrigation and I was extremely careful with how I went about it for teh first few times.