Looking for suggestions. Cautious to screw up balance and simplicity.
Tank looks great, it needs time to grow in, as you say the plant sp. you used are slow growers, if you plant it sparsely with only epiphytes you need time, even with co2 you at least need a year or maybe 2 years to get it all grown in and get that matured natural look into it. Now the plants still look just glued on, this setup is still brand new looking, not aged, but it will in time..Wait till those anubias rhizomes start to creep and form roots drapping down from the wood. Same for the bolbitis also just needs some time to addapt and develop a heatly rootstrucure once it has that it realy takes off and gets pretty large.
a major improvement imho would be hungry floating vegitation and since you are not into south american theme with this tank, i would go for something like
Hygroryza arista which has a rather large floating leaf size and a very decorative rootstructure hanging down waving in the current. Its also a hungry plant, likes a lot of nitrogene and potassium, ideal for pooping (dirty) discus tanks where poop and bioload by feeding protiene rich food for the discus (ammonium- nitrite - nitrogen levels) are always a consern for the discus keeper.
So this would be a win win, not only an improvement for the looks but also for the health of the tank water and fish, they would also appreciate the cover it provides.
The major consern for having discus in a planted tank is the dirt accumulation at the substarte with floaters you do not have this issue but still the plants benefits.