KH matters
It took me over a decade in the hobby before I was willing to try keeping fish for which I was required to alter my tap water paramters. This is much easier to do when the goal is harder water and/or higher pH. Lowering is a bigger challenge. The below is not advice to lower your pH and/or hardness, but is just what it takes to do it.
If one is trying to lower the pH of their water then KH is the most important parameter. KH is what holds pH up. So the road to lowering one's pH is paved with KH. KH also contributes to conductivity/TDS.
I drop my pH and TDS by using a combination of things. The first is that I make my own RO/DI water which gets niex with my tap. However, at the last CatCon I could not connect my portable unit to the hotel sink to make pure water. I had to make an emergency run to a nearby supermarket to buy 24 gallons of distilled water. It is pretty much the same thing as my unit outputs but more expensive. So if you want to play around to see if you might want to get a unit, distilled is what you should use.
Softening one's water means removing and/or diluting. I use TDS meters and then a continuous 3-way monitor for conductivity/TDS, temperature and pH to measure it all. When it comes to spawning for soft water fish, GH is indeed the focus. But this assumes that the pH and temp. are in the right range as well.
Finally, once you alter the parameters in a tank from those in your tap, the replacement water going in must match the tank parameters. I have to pre-batch the new water every week.
I am lazy at heart. I always want to find a solution that works which requires the least effort. The simplest solution is usually the best one.
The chemistry involved with paramters and altering them is not simple. Things all interconnect. My favorite place I like to send people so they can get an idea about water parameters and the ways to alter them is here:
https://fins.actwin.com/mirror/begin-chem.html