Wlecome to the club!
As you are just starting off I'd suggest not getting any fish that will be more than 2"-3" as adults. If it were my tank a Royal Gramma and Azure Damsel wold be my choices to add to your 2 Pajama Cardinals. I would not add any of the ornamental starfish like the Nardoa, linkia or Fromia, they do not have good reputations for survival and rarely survive more than half a year, likely because they starve to death. For a 20 gallon I would choose some hermits and small brittle star species for clean up crews.
FWIW there's no such thing as "perfect" water parameters and as there is a wide range of opinions it's better to post all the parameters you are testing for. Also, in the 4 decades I've been keeping reef systems I'd say the most important things for success are water changes (20% - 30% per month) and keeping the "big four" - Alkalinity, pH, Clacium and magnesium realitively stable.
Here's some links you may find interesting and hopelfully informative:
"Coral Reefs in the Microbial Seas" This video compliments Rohwer's book of the same title (Paper back is ~$20, Kindle is ~$10), both deal with the conflicting roles of the different types of DOC in reef ecosystems. While there is overlap bewteen his book and the video both have information not covered by the other and together give a broader view of the complex relationships found in reef ecosystems
Changing Seas - Mysterious Microbes
Nitrogen cycling in hte coral holobiont
BActeria and Sponges
Maintenance of Coral Reef Health (refferences at the end)
Optical Feedback Loop in Colorful Coral Bleaching
Optical Feedback Loop in Colorful Coral Bleaching / Curr. Biol., May 21, 2020 (Vol. 30, Issue 13)
DNA Sequencing and the Reef Tank Microbiome
Aquabiomics: DNA Sequencing and the Reef Tank Microbiome
Richard Ross What's up with phosphate"
What's up with phosphate? by Richard Ross | MACNA 2014
15 Answers
How 15 Answers Changed the Way I Look at My Tank Forever! 10 Months of Biome Cycling 12 Aquariums.