All UV is preference there is no way. If it was mine it's all the time.
In my opinion the quickest way to have growback is to change water but leave particles of this still within the sandbed. Source water seems strongly at play here, non zero tds source water even though it's unreported. Undoubtedly nutrients above normal are getting in the water column somehow, under full production light mode.
The lighting component of this outbreak is huge, lighting needs to be blued and or downplayed altogether, following a good tank cleaning, then with UV what else can we throw at it except things out of the medicine cabinet. Regarding the uv, to any degree it was not oversized to your gallonage is the same degree it may not help. Oversizing is a nice way to compensate for flow rates etc if those are not accurately being factored in uv sizing.
In a lab setting, flow, maintenance and sourcing of the bulbs is superb and doesn't have to be oversized. Lab quality uv was source feeding our whole lab at a large beef processing plant I used to work at, we did lots of pre and post wastewater uv measure they 100% work on some targets and your suspended algae sure is a potential one. Not hard to overdrive photosynthetic machinery and get radical meltdown off a pass through of blasting light.