WOW Marc . You have a whole house invested in your reef equipment
.
The logics in discus tanks are completely opposite from the reefs . Your described start where you build your system very slowly adding fish by fish is not used in discus.
My 2 cents would be to buy your discus batch of let's say 10-12 fully grown discus all together and let them settle for 2-3 months in your bare tank . You will use this time to learn their behaviour and their needs. You will learn when and why they are happy and when and why they act odd .
Once you and your discus do this then you can put sand , potted plants(pre - sterilized with bleach ) and driftwood to enjoy your tank at fullest .
Then , you can also add other diether fish in big schools , (You'll need 100-200 tetras or Cardinals for that size tanK so they can look cool IMO) but only after you perform the strict Quarantine procedure with tester discus fully described here on out forum .
And wolla , in 4-5 months from now you got your self a breathtaking discus planted tank .
Your system holds total of 210 Gallons of water . Your 2 drums of aged water holds 120 g and that would be totally enough for you to perform 1 or 2 - 120gallon changes per week (that would be 50-60% change ) .
You don't need anything else IMO to have a beautiful discus display tank and you can sell the rest of the stuff from your ex hobby , reefing that is .
Good luck and ask away if I have missed something from your long post .