Going to throw my hat into the ring here....my thoughts on this enigma are...
Temp is too low. The average - if this were my aquarium - would be 75 and not 73....two degrees can make a lot of difference when health is a question.
pH is not compatible for the mix of fish that you have..the Cories will handle a wide range pH and your's is at the top end of the range, Cherry Barbs again top end of their range if a little above it, BN's are OK, Rainbow Sharks are a smidgen above their normal range, Khuli Loach is on the high end if not over top of their range
Planting is definitely unsuitable....you need to have multiple sight blocks (ie clumps of thick planting that will stop one fish from seeing another) absolutely vital when you have aggressive fish such as the Sharks. Also things like caves, wood, rocks to act as santuary for bullied fish (just cos you might not see bullying does not mean that its not happening as much of it is at night/lights off). You should also have a sandy area for the Cories to sand snuffle and the Kuhli to burrow into.
I suspect that your fish deaths have something to do with the combination of stock, excessive bullying by the Sharks and your lower aquarium is overcrowded, species wise, and they are fighting over food. Despite looking like they are quite tough, the Cories are not great at learning about territory and since the Sharks are hyper territorial there is every chance that the Sharks have literally harrassed and chased the Cories to death. It happens more than people realise. The deaths of your Cories will be due to a multitude of things, a perfect storm of water chemistry, poorly chosen/aggressive tankmates, insufficient planting/scaping to allow fish to hide and relax, substrate not really being right and just too many species all fighting for space and food on the lower regions of the aquarium.
I would suggest that you decide what you want to keep and rehome the rest.....such as rehome both of the Sharks and one BN. I would then add a few more Khuli's since they are not lone fish, they like company to thrive. I would maybe add a few more Cherry Barbs as they tend to get nervy in small numbers.
Your top priority is rehoming the aggressive fish and a BN, the water chemistry, the filter servicing (no need to change filter pads monthly, wait til they fall to bits), the planting and aquascaping. I would also suggest you purchase a small
air pump, airline and airstone cos thats alot of water without an ancilliary oxygen supply.....and the regular surfacing of Cories is indicative of an oxygenation issue.
JMO.