Boondoggle,
Can you just take a long stick with a little fork or small branches on the end and sample for FA below your secchi disk depth?
I would sweep it around down to about 3' deep where you have some firmer substrate on the bottom and maybe also sweep through the edges of your brushpile. If you get FA on those sampling efforts, then you know it is coming.
OTOH, even though everyone hates it in their fishing pond, you might just observe your FA cycle for a full year since you aren't fishing yet. Save your budget and time, and then go full bore next year if that is needed?
