Various plumbing parts: (schedule 80 bulkheads, gate/check valves, unions) GFO/carbon reactors.
Maybe a battery powered airpump, sponges for a diy emergency filter if the power goes out, which can double up as a filter for a QT.
although minor in the grand scheme of things, acrylic for your external overflow box (my understanding that low profile means the piping is outside?) little things like bulkheads, extra unisons, weld-on 4, diamond coated drill bits (assuming you dont have them) adds up quote a bit of $$
grow lights for your refuge and maybe a fan for your cabinet. emm.......fish/corals?