I've had a shower drain that occasionally drains slowly and often has sewer smells. I've done the usual 'home remedy' tricks (vinegar/baking soda, plunger, etc) which have helped with the drain speed, but the occasional sewer smell remains. Note that when I say occasional here it's probably 2-4 days a week. I wouldn't say it's associated with long periods of disuse (and water in the trap drying up); if anything more associated with heavy usage of other plumbing on the same vertical.

So just now I took the shower cabinet panel off and looked underneath. I'm pretty sure this is just an insufficient / ineffective trap. It's just a waffle-type tubing that goes down to floor, along the floor for perhaps four inches, then up and down into the drain pipe in the floor. (I'll try to post a photo later). I'd guess that the max trap height is two inches, possibly less.

So it seems to me that this is just plain insufficient and there isn't a proper water seal (is that the right term?) in the trap to keep gases from coming back up, and will likely need a different type of trap. Does this sound like the right diagnosis? Any suggestions?

As this is a shower cabin elevated off the floor, the main constraint would appear to be vertical space from the shower drain to the floor drain, and there is a fair amount of space under the cabin in other directions. I think putting any kind of trap further down/under the floor will be very difficult and costly - this is a concrete floor construction.