I run nothing but canisters and HOB. My best success is running two canisters in series; one as mechanical & chemical, the other completely filled with nothing but biomedia (Seachem Matrix). Since you already have AIO filtration I second the motion to add a canister solely for the purpose of added biological filtration.
Personally, I can highly recommend the Oase BioMaster Thermo 600 which will give you one integrated 300w heater, a very easy to service, removable prefilter, and you can fill all the trays with Matrix. You could step down to the BioMaster 350 but I would not go below that model. You cannot fathom just how practical, useful and convenient the removable prefilter is. Purchase the $16 Carbon Pre-Filter Foam Set, instead of whatever pre-filter foam it comes with. I've run them all in both fresh and saltwater, and the carbon foam is the absolute best balance of particulate filtration without choking at the surface.
If you want to add on a canister as cheaply as possible, something like a SunSun HW-704B can be purchased open-box for around $80, or the SunSun HW-3000 is back in stock and selling for a reasonable price of $130. Both have a 9-watt integrated UV which IMHO is nearly useless in saltwater. It will kill algae but won't touch parasites at the filter's lowest flow rate.
So the jest is...
Run your AIO setup primarily for mechanical and chemical/carbon. Add a decent canister for biological filtration and you should have a pristine system.