You mentioned that the stuff you pull out is very smelly. Just wondering, what is this stuff? You mean shrimp waste, or is this perhaps dead plant and/or algae matter? I'd be surprised if you have so much waste from the shrimp as I can't detect any from my 15 or so critters, so even 2 or 3x the waste would still seem less than you're talking about. I only have moss (Christmas moss I believe) in my tanks which I love because it is so low maintenance and there is virtually no decaying plant matter to deal with.
40% water changes each week seem big. I know RCS are at the hardier end of the shrimp family, but they still need stability. If you're having deaths, maybe give some thought to if the big swings in water parameters with big water changes could be an issue. Honestly, my tank really came together once I started to RO water, switched from Equilibrium minerals to Salty Shrimp (much lower TDS), and reduced water changes to less than 10% at a time. If you need more water changes, better to do more small ones than one weekly big one. But ideally, the fewer and smaller water changes the better to keep everything stable.
Also, snail poo = shrimp food. My tanks are (unintentionally) full of about 10-15 Malaysian Trumpet Snails each. I like them so they're welcome to stay and keep the soil aerated, plus give the shrimps something to graze on. I'd definitely recommend some sort of snail in a shrimp tank.
Have you come across Marks Shrimp Tanks on YouTube. Scottish shrimp keeper living in Norway. Worth watching his videos of water changes, feeding, tank setup and maintenance, etc. Some good pointers there for sure.
Hope this helps anyway, but fingers crossed an expert will be along shortly to help set us both straight!
Also meant to say, maybe consider 'upgrading' the filtration in your tank. Seems pretty decent already, but maybe gradually switching the bio-media to Seachem Matrix might help boost bio filtration. And adding some Purigen will help remove organic matter before it has chance to decompose, hence reducing the need for big water changes. Just a thought...