I am confused why your HC is not taking off either! You have decent substrate, lighting and you're running CO2. Are you pumping your CO2 up til the drop checker goes lime green? It's fine if it drives your pH down to 6.8, just make sure you do it very slowly so that your fish can acclimate.
I used to mess with many different substrates and have found the most success with ADA Amazonia NEW. I don't know if this advice helps you, but if you are super committed to getting that HC mat in, you could pull your water down til about an inch layer is left, pull out the HC, take off an inch of soil, gently pour amazonia over those areas with a cup til u get a good 1/2"-1" layer, then replant. People are gonna say I'm crazy, but I swear by this stuff now lol.
I recommend messing with substrate before going with dosing because I HATE HATE HATE dosing. A lot of it has to do with bias and frustration with it, but I honestly hate playing master chemist with a living ecosystem. I don't like dropping and dumping different additives and chemicals trying to get some balance that will facilitate growth. If you don't know exactly what you're doing, you end up wasting all sorts of money on different additives, or worse throwing something way out of whack that kills some fish. I like to keep things simple and minimalist. Mess with the basic elements: soil, CO2, light. Perfect those 3, and forget about the rest. The exception to this rule is if you are working with special case plant species that DEPEND on certain additives. HC is DEFINITELY NOT one of those species.
And once you have tweaked those 3 elements, the ecosystem runs well by itself with minimal maintenance. If a system grows to depend on you dropping X and Y into it every week, pH up or pH down chemicals, ammonia or phos reducing chemicals, etc...that's not very stable, is it?