How often are the fx4's being cleaned? if only monthly that is the problem.
monthly water swaps? how big?
I have ran saltwater tanks on canisters and was just fine but you need to maintain them more often. even every two weeks may be to long for a canister but would be a start. the food and fish poop that gets caught in them starts to break down after only a couple days, beneficial bacteria does its job and converts it.
I would clean the fx6 on my 75G weekly, it fit 5G exactly so that is how much of a water change I did.
If you could pull one each weekend, rinse filters in the old water and get all the crap out, replace with freshly mixed salt water and put back. Then the next weekend do the second one. but still would suggest 10% water changes bi weekly on top of the water going into the canisters.
what are you using as a skimmer? i think that is the biggest benefit of a sump is being able to have a large skimmer.
sump needs to be as big as the equipment you plan to run to help with your nitrates and phosphates?
skimmer, refugium, algea scrubber, cryptic fuge, heaters, return pumps.
need more of a plan. i shoot for 30-40% of my DT volume as a sump. so if i had your tank i would be looking at a 65-85g sump, maybe a 60 breeder would work because it is shallow and give a good foot print.
but even with a sump and big skimmer you still should be changing sock filters every 3-4 days.
what maintenance do you do between their visits? just adding a sump is not going to fix anything if the maintenance aspect does not become more frequent. you will just spend a lot of money on pumps, plumbing and equipment and have the same outcome.