This sounds like a fun project
with upgrade plans I'm always slightly wary especially with big fish, as simply put life happens sometimes and they dont always happen.
Stocking wise you do have a really broad range of options - probably slightly frustrated by these lock downs though. I like to keep threadfins with Geophagus I had mine with Winemilleri and Red Heads and all did well. You water is nice and soft for them so should avoid issues I faced - susceptible to disease and potential hole in the head in harder water. You could go a size down to Guianacara or even smaller to Cupids. There are a mix of Acara species that could work, blues, yellows (metae), Diadema, Krobia Xinguensis.
I dont think I'd go for any of the bigger cichlids here like True Parrots or Chocolates I think you have enough bulk with the Sevs
What do you think to some of the easier to keep Pikes? The dwarfs are pretty steady going but if you wanted something bigger Saxatilis has a good reputation and (if you have the money) Zebrina are stunning.
I always think its important to balance out south american tanks with characins, catfish and oddballs. If you wanted something for the surface Silver Hatchetfish are a good option as they get a decent size. Mid sized tetras like Lemons, Flame or if you wanted something larger Bleeding Hearts are nice schoolers, Splash Tetras are hugely under rated fish IMO and some get to a surprising size so worth looking into. Headstanders are interesting and some really interesting species available now if you can hunt them out.
For catfish I prefer Whiptails to Plecos but the fancy L numbers are pretty amazing L95 is fantastic but there are a bunch of smaller ones too - been quite into Blue Baryancistrus recently. Whiptail wise though Lamonicthys, Sturisoma are cool
For bottom dwellers there are a range of options I've had flagtails and dwarf hoplos in the past but plenty of cool Cories and Brochis around now too. Always wanted to do a school of Cories with some rogue Brachyrhamdia mixed in to try and witness the natural behaviour. Wood cats are a group often over looked, some are totally invisible once in a tank and just hide 100% of the time but some species are pretty active. I had a couple of small Ageneiosus that stayed in the top corners of the tank all day just hovering facing the sun and then as soon as the tank lights went out patroled the entire tank looking exactly like a reef shark which was really cool.
One thing I would say is that it would be good to switch to a much finer sand substrate then you have right now as it looks a bit too course for your threadfins to be filtering as an earth eater
Wills
