I think you may have slightly misinterpreted the material you read about KH and clay-based substrates. While it is true that most soft/blackwater aquariums (and by extension most planted tanks) should have a fairly low KH- I aim for a KH of 2-3, what you need to watch out for is that baked clay substrates, “aquarium soil”, turface, etc. all strip hardness from the water. This is an attractive feature of the substrate when you are using harder water from the tap, but must be managed when using RO water. If you don’t remineralize the water you can end up with a KH of zero, which can lead to significant ph swings caused by co2, tannins, and other organic acids commonly found in planted tanks because the water will not have any buffering capacity.
This is easily addressed thru regular testing of KH and appropriate re-mineralization of ro water, cutting ro with dechlorinated, preferably carbon filtered, tap water, OR (and I would only suggest this if you test often and have some baseline for the effect it will have on your tank) by adding a few rocks, aragonite, or shells that do leach small amounts of carbonates in to the water- I’ve only done this in larger tanks where the KH tended to run close to zero, and where the relatively minor buffering capacity it afforded me did not raise KH to the point that it was counter productive.
Like everything else in planted tanks the key is stability and balance. Active soils will by their nature strip KH from your aquarium water, but if you test your KH regularly and are aware of those properties, you can easily amend your water to keep the KH at the ideal range of 2-3. The rate at which your substrate pulls KH from the water will lessen over time, and usually levels off completely after 1-2 years depending on the substrate, so you’ll need to test continually to make sure you’re adjusting the water’s hardness accordingly.
Initially I would re-mineralize your water to a KH of 3 or 4 to help offset the KH stripping properties of the soil, and drop that down to 2 once the soil has aged and is less actively stripping KH from the water. The key is regular water changes of 25-50% a week for the first few weeks until things level out. This will also help keep ammonia from spiking to dangerous levels. After a few months, the tanks inherent KH lowering properties will begin to level off as the system stabilized. At that point you can begin to remineralize your makeup water to a level closer to the target KH of the aquarium.
Hope this helped!