haha, substrate can be a pain to select sometimes.
I recommend you buy some Miracle Gro's Organic Choice Potting Mix. Use that in your tank as the bottom layer, and then cover it with play sand. you would have to plant your plants right after you set down the potting mix, then cover with sand.
The soil will provide your root-feeding plants with enough fertilizer to carpet your tank and grow very well. The soil also to some extent will supply carbon for the plants that have access to it (once again root feeding plants)
If you are going with 2.5wpg or more, i would recommend you do not only excel but DIY CO2 like you planned. go easy on the excel once your DIY CO2 is running to avoid carbon overdose.(not sure if its entirely possible, but its best to be safe)
I had a 10 gal high light iwagumi setup once, but it crashed after a while. they key thing I was missing is constant CO2 injection(I was lazy about changing by DIY reactor bottles lol)So just make sure you regularly replace them to keep your CO2 levels constant. I attached a pic of what it was like before it took a turn for the worse. I used eco complete substrate there and had 4.8wpg of T5HO light. without CO2 it was disaster lol. But like I said, the soil with sand substrate will work really well too.