On the "yellowing" you mention, that is a nutrient issue that substrate tabs on their own will not solve. Some nutrients are taken up via the leaves (potassium, ammonium, as well as CO2) so a good liquid fertilizer is best. Flourish Comprehensive Supplement
mentioned by prior members is what I use, and there is also FlorinMulti by Brightwell Aquatics.
On the algae, that is caused by light beyond what the plants can use because nutrients are missing. The liquid fert will help, and you might consider reducing the duration (haven't mentioned yet how long the light is on now). I control brush algae with light duration.
The CO2 would give you faster growth obviously, provided all other nutrients are increased to balance, along with the light. But the first photo is beautiful, I see no reason to go down this road when the tank is this lovely.
The state of the Echinodorus major
in the second photo intrigues me; mine is doing much the same, after a couple years of incredible lush growth and even producing an inflorescence with flowers (very rare) and adventitious plants (also rare for this species). I'm not sure what is wrong in either case.