Just set pH on the back burner in this instance. kH and gH are what you should focus on - hardness, osmotic pressure. pH really just helps give you an idea of what your kH is and whether your buffering substrate is outta whack. Once you have a liquid test kit for hardness, shrimp folks will be able to give you better advice.
40% water changes aren't inherently bad - especially if parameters are matched or new water is slowly added to the tank. So that's probably not the issue.
24c is a bit too warm for most Caridina varities. Higher temps than they're used to can on occasion introduce bacterial and pathogenic problems they wouldn't otherwise experience or haven't experienced previously. I've only ever had issues with Caridina species when temperatures jump above 22c.
kH 3 is likely too high for them and is probably why you're experiencing issues. It's extremely difficult for adult shrimp to adjust to osmotic pressure extremes compared to juvenile shrimp. That's why it's important to have a good idea about your shrimp's source water, as @Zoidburg
TDS is just all dissolved solids in your water. Knowing that number won't be of any help at all unless you know specifically what you're adding to your water and how it impacts your parameters - like hardness.
That said, if you're fertilizing a lot, it's best to slowly acclimate shrimp over the course of several weeks or a couple months.
What are you feeding your shrimp? It's possible they're getting more protein than necessary (or more than they're used to) and that can compound molting problems along with hardness.