Well...cyano seems to be going away.
I initially added about half the gfo that was recommended. When I added more 2 weeks later, I probably should've just put fresh gfo on top. Instead I threw out the old media and probably added too much fresh media at once. My zoas looked unhappy for a couple days, and a couple sps lost a touch of pigment at the base within 2 days. I cut the amount of gfo in the reactor in half to correct the issue, and added a dose of aquavitro fuel to the tank. Also fed some reef chili a coupe days later.
A few weeks ago the reef chili and fuel combo definitely would've made the cyano spread like crazy...but none seemed to come back, so that's a good sign. It's still there in places that I didn't siphon or blow off the rocks...but those areas haven't gotten any worse.
I think what probably helped most was that I stopped being lazy. Grabbed the turkey baster and blew all the rocks off every few days (as opposed to every few weeks). It looks like I had some spots where detritus was allowed to settle and build up. I cranked up the flow on the mp10 from about 70% to 90%...I hadn't considered it before, but the added sps seemed to block some of the flow I was getting through the tank when it was less stocked. Things are now back to where that top layer of sand is getting pushed around nicely without getting kicked up too much and circulating.
Hopefully I'll get to add cyano to the list of nuisances/pests I was able to keep in check. Hair algae, bubble algae, and aiptasia have all tried to knock me down and lost.