Hmmm... I just went through something similar a few weeks ago. The thing about that type of odor/gas is that it will rise and linger depending on air currents in the room. You may not be smelling it near where the source is. I will tell you that with our shower, we have been squeegee-ing the walls, and rarely use cleansers in the shower, and I have lots of thick hair that can get stuck in the drain (but not clog it) and what I found was that after I had cleared a big hair clump, the actual drain trap was stinky. I had been smelling it for a week or two- it smelled like sewer gas; I poured 1/2 cup of Clorox into the drain trap, let it sit overnight, and now is 3 weeks and haven't had it again. I think my trap simply got funky. Another time a plumber didn't put the house trap on tightly and one corner of the basement was where the gas collected... 10ft from the source. So look around, not just where the odor is, cause that kind of odor is rarely sitting right at the source! And in the event you haven't been using that shower, make sure there is water in the trap (which prevents the sewer gas from rising out of the drain).

Take of the grate in the shower floor and clean the drain thoroughly. A large bottle brush will do the trick. It sounds like crud has built up above the trap seal in the vertical drop. Hair and soap buildup will smell after a little while if not removed, even if you treat it with bleach.

That would be a fail there, dealing with the flex tube, which would cause constant clogs, how does it connect to the real drain line? Looks like it just terminates there and there is nothing to actually seal the connection. Probably have constant sewer gas leak there. Surprised you havent had water leaks either. I understand the difficult access and the flex tube is probably all they could use, but they really would need a reducing fernco to seal the rigid and flex. Expect to continue to have clogs with using that flex. And it is technically an S-trap which might siphon water out of the trap causing sewer gas. If it is possible replace the flex with rigid, should be enough space for a P-trap. Then there is the venting and S-trap issue, connect the drain to a vent before it goes down using a sanitary tee. Just a lot of fail here, and hard to fix without access to a proper vent.

We had to remove an anode rod from our water heater to help with the sulfur odor.