The theme you are hearing is a pretty consistent refrain amongst hobbiest and the genral concensus seems to be (which I agree with)... You need a skimmer, do routine water changes, get rid of the canister filter and HOB Whisper filter (with the one exception that some use them to run Carbon, but don't use as your primary mechanical filter), you are probbably going too fast... adding livestock faster than you should (seems most of us do when we start) LFS often give really bad advice and sell you inappropriate livestock, a lot of us have reliability issues with test kits or like myself, have enough color blindness that it makes reading them very difficult, use RO/DI water with very little to no TDS, for fish-only lights don't matter much at all but matter ALOT for coral, livestock as gifts are routinely poor choices, got to have good water movement/flow in the tank although this is far more important with coral than just fish, , and the sticky's at the top of this forum will answer pretty much everything you need to know to get started. Your KH and nitrate are a bit high but not enough to be a significant problem IMO, especially for fish, and that alone would not explain death of fish.
Yes, for every point where there is reasonable concensus, there is an exception, someone who has done it differently with success. I think you either read a lot and follow the concensus, or just find someone you really trust and don't listen to everyone else who you have no idea if they know what they are doing or not.
Like others have said, we all seem to have experienced unexplained sudden death syndrome
Sure, you can use a HOB skimmer in a sump so it's not like you are wasting money using one now and moving it. I have used the AquaC Remora product on several tanks and had a good experience with them. They used to be wildy popular but not so much anymore. you will hear a lot of support for the Reef Octopus skimmers here on RC. I have not heard of there being an issue with overskimming or using a skimmer too large but as a practical matter, I am not sure there is a lot of value in oversizing too much though.
Anyway, those are a few of my thoughts