I think that you have 80-100 PAR at ~10", so I'd tend to want a short photoperiod initially, as @Ddrizzle
The smaller the tank, the more sensitive it is to variation. This may or may not be the solution, but I would try the following:
Reduce the water change amount to maybe 10% twice a week until things stabilize, A 50% water change is going to give you a big CO2 variation once a week, which is ok when you have a good, stable plant mass. The Aquasoil is going to throw parameters off for a while, so you may have to adjust water changes to compensate. Again, the watchwords for parameters are: stable, stable, stable. Initially, you would also probably benefit from an Excel treatment to tamp the algae down until you have a month or two behind you. I'd do a half ml/gal right after each water change.
You are probably overdoing it with the CO2, as your Betta is telling you. It's more important to have stable CO2 than high CO2. A 1 point pH drop should be plenty and I'd run it 24/7.
Test for NO3, PO4, GH, KH and TDS if you can. Between your dosing and the substrate you may have balance issues that would need to be brought under control. The Aquasoil may eliminate the need for some dosing.
Good circulation and gas exchange are also important.