I'm by no expert, just sharing what I have and been recommended.
I have a similar setup to yours 7.5Gal tank with 1 betta, 2 amano shrimp, 4 rasboras, 3 otocinclus and many MTS.
I have Ludiwgia and Miriophyllum with some Baby Dwarf Tears, Anubias
Sand substrate and use Root Tabs.
From what I've read, CO2 is one of the main sources of food for live plants.
Just like O2 is needed for fish, plants need CO2 so that they can do Photosynthesis.
There are two main ways to supply CO2 that I've seen, one is through DIY and then Injected.
I use Injected and started with a minimal setup (manual) to learn and try it, but quickly became a hassle because some consistency is needed, so I had to be there to either turn it on or off and then try to get it to the right amount needed.
So I invested on a Solenoid to get the right amount and on a small timer to get the time consistency, and since then the plants grow very fast.
Also good circulation is needed, otherwise CO2 just escapes.
To determine the right amount of CO2, you need to test your water for PH and KH and use a table that tells you if you are in the right range.
One thing to make sure, is that you have good water surface agitation, so that gas exchange can occur and your water gets enough O2 for your fish.
Airstones done add O2 to the water, they just provide a way to break the water surface so gas exchange occurs.
This is important because on the absence of light (at night), plants will instead consume O2, so you want to make sure there is plenty for your fish as well.
One big note:
whenever you introduce plants to your tank, make sure to check them and clean them throughly, you don't want undesired algae.
I made that mistake and now am battling trying to get rid of it.
(Just for future reference in case it happens to you: plantedtank.net: What kind of algae and how to fix.
, they are awesome and very knowledgable)