A Ph of 7.4 is fine to keep many soft water species: certainly most South American tetras and corydoras species.
For the more sensitive species, however, like discus, geophagus, Uaru, and many of the South American black-water species KH/GH are a far more important number to consider.
So, Im wondering...
If 1 dKH is about equivalent to 17.9 ppm- isnt your KH and GH very low? I usually think in terms of General Hardness(GH) and Carbonate Hardness (KH)- not ppm.
OP mentions a night of degassing, but Iím not sure what is being degassed if CO2 isnít being injected. I would think pH would be unchanged. Maybe just chlorine/chloramines?
Youíre right about the KH readings being very low. pH is determined by CO2 and KH. If KH ~1.2 (20 ppm), there is very little buffering and OP is likely to see pH drop if water changes and cleaning regimen arenít good. CO2, from atmosheric source should be about 2-3 ppm. With a KH of 1.2 and 2-3 ppm CO2, pH should actually be about 7.1-7.2. As you know, a pH pen is better at finding pH than the reagent tests. My guess is that KH is higher than the OP suspects. Of course, some water companies add sodium hydroxide to raise pH to prevent acidic water from damaging plumbing, which throws the calculations out the window. Iíd ask the water company if they do this.
To test low KH levels, I use five times the recommended water levels for reagent tests (such as APIís) then divide the results by 5 to get a more accurate read. If we can be sure of KH readings, youíll be better able to assign fish to appropriate levels and to project pH more accurately on an assumption of CO2 being in the 2-3 ppm area..
Concerning water softness, I would look at 4 dGH (75 ppm) as being borderline soft, but not much higher. Might also be useful to get a TDS reading, but letís make sure about the TDS meterís scale (.5 or .7) - probably would be .5. I view TDS as being a better guide for my fish, but I donít get into the more sensitive species that you do.