I don't have a sump either, all of my various forms of filtration are provided by a canister filter, live rock, an HOB protein skimmer, and a sulphur denitrator. Because of its aforementioned nitrate producing capabilities, I remove most of the mechanical filtration from the canister filter and use it mostly for it's activated carbon and biological filter. (live rock is my other bio filtration). Also, since I'm doing a mixed reef I don't want too much mechanical filtration since that is counterproductive to what filter feeders need.
FWIW I don't have a nitrate problem, my 'trates are under 5 ppm. I do infrequent water changes (4 times a year?) but am able to continually export nitrates by using the sulphur denitrator (Korallin S1501), which turns nitrates into nitrogen gas which just floats out of/exits the tank naturally. I could set it to keep nitrates at undetectable levels but corals like at least a lil bit of nitrates so I don't have it running as efficiently as I could, plus I intentionally overfeed just a little bit to keep the corals doing well in otherwise relatively sterile water. I change out the carbon about once a month and rinse the bioballs in outgoing tank water at the same time. Other than dosing Alk and Calcium and Top-Off Water (and feeding of course) it's a fairly low-maintenance tank. All of my corals appear to be doing well, and I've got a variety of types: an SPS, an LFS, and a Softie. Color and polyp extension are great. Haven't noticed much growth yet but it's only been 3 weeks since coral was added. Fish and inverts have been doing great for months, ever since I dialed in the denitrator.
The point being, my reef tank appears to be doing just fine without a sump. And I have more peace of mind when leaving home for a few days because I don't have to worry about a power outage causing a flood in our house. Sumps sit below the display tank, water flows from the display tank to the sump via a natural siphon, but water is returned to the display tank from the sump using a water pump. If power fails, the siphon continues filling the sump, but that water isn't pumped back out of the sump any more, so the sump just fills, and fills, and fills. People with sumps get around this by having power back-up systems, but you said you're trying to keep time, effort and expense to a minimum, so this doesn't sound like a good option for you.
If you did decide to go with a sump with your display tank in it's current position, with some creative plumbing you could put the sump in a different room altogether. What is on the opposite side of that wall behind the cabinet, for example? You could theroetically put the sump in that room and then plumb through the wall to your display tank, but again that's much more time, effort, and expense.
The negative with my setup, of course, is that HOB protein skimmers are rather unsightly to look at. But no one notices those things anyway, they're always too captivated by the tank.
