I also just confirmed that overwatering can cause P deficiency. Watering when you feel it is dry where? The top can be dry and the bottom wet.
Here is how you do it. Fill a pot of the same size with FFOF, lift it, this is your dry weight.
Now lift your pot, does it feel a lot heavier? Then it does not need water. I found with smart pots, during the winter I needed to water every third day, regular pots maybe every 4th or 5th.
The hefting method is the only way to go.
Some people recommend those stick pH meters but I have never seen an accurate one. There are some decent pH pens out there for under 40$. Spend a little more and get a ppm (TDS), and pH in one.