Whoa. Something is still a little off. New growth seems acceptable but it also seems like you're having trouble sustaining the old growth long enough to allow you to multiply your plants. It's like plant purgatory.
To attempt to answer your question, have you measured gh/kh recently? I'll tag @Seattle_Aquarist
to chime in for more analysis.
Did you ever measure that co2 properly?
what's your micro regimen like? U mentioned uns liquids but not much details. Which bottles so that we may help with the amounts. Might be something to look into as well. My final "hump" of getting to happy growth required some attention to the micros.
What's your lighting period? I feel (no science here
) that it's a little too long for the current state of your tank. Theres been good arguments of late that strong light alone does not cause algae so I'll refrain from saying that straight out, but it was accepted that if your tank state is not optimal, it sure accelerates the algae mess and will easily turn a bad situation worse. So think about shortening, give the plants and yourself a little less stress while figuring out what's missing.
Also, the community of late is accepting to the idea that full ei + rich soil substrate + a smaller tank may be an overkill situation. Even if it's not the direct cause of your problems, it may not be helping. There's thoughts about one fertilizer obstructing the uptake of another.
And finally, I threw all the ideas out as brainstorming. Might not want to enact all the changes at once!