Tank is about 5 months old.
Nitrate 20 ppm
temp is 76
What you have looks good although as Nubster pointed out, kH and gH are very important when raising shrimp.
gH will help you figure out if they are having molting problems (gH 6-10 is usually a good range to sit in) while kH will help stabilize your water against pH shifts (kH 3-5 is a pretty good range).
That said, LFS shrimp tend to be a little more fragile than homebred. There's a good chance they were imported from overseas, which adds a lot of stress, and even if they weren't, most LFS don't have shrimp-specific parameters so the shrimp have been accumulating stress there
as well. Finally, they are purchased and moved to another
tank with potentially different water parameters. After all that, some of them just don't make it.
Keep an eye on the juveniles and get a kH and gH test kit (a $15 TDS meter will help you in the long term as well). If you continue to see deaths, report back.