Welcome!! You are like the tenth person this week I've had the pleasure to congratulate for coming here before they get discus. I wish I was so fortunate !!!
So far your plan sounds decent. How deep is your sand bed? How were the current filters cycled ?
What are your water params currently in the tank? You have tested the tap water and aged I 24 hrs w an air stone and re tested it? If that's the case and you have no swings you can match the temp and do direct from tap water changes.
Sounds like you have a lot of decor. Is there swimming room and all? Are the fake plants rough or sharp in anyway same
For the wood ?
Size of discus is a very personal thing and depends on your wants and budget. Some folks love seeing the fish grow and like to buy smaller they are also cheaper. Some prefer to let others do the hard work and pay more for grown fish.
Bigger is always better for newbies because they are usually healthier and more developed and able to fight disease better.
Since you are new I'd buy the biggest you can afford go to 5" if possible.
Your water change would do much better at 50% or higher. But again depends on the size of the fish your feeding schedule and the bio load.
Lastly. Folks will tell you that you will already take a risk adding discus because you have cories in there already. If you add cardinals then discus. Well now you have doubled your chances of cross contamination. This is something to consider and takeinto account.