I had a similar problem a while back. I had very dirty substrate, and my CO2 wasn’t stable throughout the day. You might want to give your gravel a deep vacuum and see what comes out. Judging by the first picture the substrate near the glass looks dirty, probably throughout the tank. Do the vacuuming in sections, uproot the plants, vacuum 20% of the substrate, then do a large water change. Maybe spread it out over a couple of weeks. Drop your light level a bit while you fix the buildup of organic waste in the substrate.