At least test GH/KH. Most well water will be liquid rock.
Correct me if I'm wrong (I often am) but I think those tests are better for people who want to affect change to their water in order to make their water acceptable for a specific species of fish or plant. My fish and my plants will accept my water or they'll die and be replaced with species that thrive in my water. I have already steered away from fish and plants that like soft and/or low ph water which is unfortunately why I can't ever have Discus which I consider magnificent.
I have a deep well, right at 400' because that's the best quality water on a coastal plain. I already know my water comes out of the ground with a ph of 8.2 and oozing methane. Believe it or not my water will burn with a beautiful blue flame for a second or so. Then the methane bubbles out and my water looks and tastes excellent, it's like having seltzer bubbles in your water
I also have fairly hard water but Central American Cichlids do very well in my water. I'm already learning that some plants thrive and others die, I'll keep the ones that thrive.
What really concerned me was adding chemicals. I was looking at the list of chemicals in the dry ferts and I was wondering whether or not I already have some of these specific chemicals in abundance, if I did then adding more might be counterproductive.
I noticed that I can test for Potassium, is there anything else that I should test for before I make my dry ferts? If I already have something in abundance I'll tailor my mix to my needs.