I'm gutted to hear this. So sorry, Simon.
Unless your home gets awfully cool, you might find that your shrimp tanks do just fine without a heater. I have 11 shrimp tanks and only one is heated (to 74F/23c because of one fish). They slow down breeding in winter, but I don't mind that. My tanks at my shop drop down to 66F (19C) in winter months and I've never had losses related to cool temps. On the other hand, I have lost hundreds upon hundreds to tanks getting too warm. Just food for thought and if your place's temperatures fluctuate a ton this might not be ideal especially in a smaller aquarium. But I once lost power at my shop in the dead of winter and my tanks got down to around 11c and and Crystal and Neo shrimp didn't die. They were motionless and not happy so I made sure it took days to warm back up by setting the room thermostat quite cool and inching up slowly, but that was last winter and I still have those colonies (as well as some fish in other tanks that went through it). I would not recommend such extremes obviously and realize I got very lucky.
Just a head's up you might already know about, but that Dennerle substrate will spike ammonia the first few weeks. It's probably my favorite out there, but there's quite the waiting period before it's safe to add shrimp.
Have you tested the tank parameters as that is the most important one?
The tank looks good. Have you removed all the rock? If you have and the parameters (Ph) are going down with the water changes then you have probably sorted that part of the problem. If not I would try removing all the rock at this stage (you can always put it back later?) I left my rock a week in the test water before testing so you may be being too impatient, test again in another 4 days as it won't instantly rise it will take time to slowly increase the Parameters (Ph) I expect?
Some occupants were probably weak/sick by the stage you started 'messing' with the parameters etc, and changing the parameters may have just been too much or too late for some, if they were already weak/sick?
I did the tests tonight, and URGH!
Results were useless... I left gravel and the rocks in water for 3 days, and showed the tiniest change (if at all...). I also tried the vinegar test with white vinegar, but also had no reaction.
In the meantime I have lost the rocket killie that was originally sick, and another 1 or 2 😭. HELPPPPPPPPPPPPP!
Should I try with hyrdochloric (or muriatic or even citric) acid just to be sure? And would it have to be pure?
Upside-down strip comparison (srry)
My interpretations: (note- the gravel was in much less water, water barely went over the gravel, but was in for less time (12 hours))
The rocks and gravel
And my problematic tank in all its bad blurry phone photo glory: