Yeah all aquasoil types do that. Adding extra PO4 will really make a big difference during the first couple of months. Try an extra 5 ppm all at once after doing a water change, then the normal dose throughout the week
I changed over to Landen soil, and during the first few months it sucked up PO4 like mad.
Your pic of the algae is exactly what mine looked like if PO4 ran too low. I have seen the same with many other tanks as well.
I would not only add an extra dose, but also bump up your normal dosing. Instead of 15:2:20 try maybe 15:5:20 for a few weeks. I would also take a mid week reading to see where things are. I dosed PO4 at amounts that would make your head spin in the first few months, but whenever it bottomed out things got wonky. PO4 is nothing to be afraid of, and plants love it. In this initial phase don't be afraid of lathering it on.
And remember, things don't change overnight, so whenever you make changes give it time to see the real effect.
It would also be helpful to know the approximate PAR level at substrate. Both too much and too little can be an issue.
Sounds like CO2 is good. I would make sure to create good surface agitation for aeration. Don't worry about off gassing, CO2 is cheap in the scheme of things.
And remember CO2 and O2 are not mutually exclusive. You can have high levels of each. If you have good oxygen levels, CO2 will have less effect on your livestock, and keeping oxygen levels high is just good for the tank in general.
Good luck and looking forward to seeing where this goes.