So... is there any reason not to do this? Been using boric acid in
alcohol for years, because that's what I was taught, but this
seems pretty effective, maybe even more so.
Sure it works. The flux is more active than just the boric acid, so
it will tend to burn off more quickly if you heat for too long, but
for just annealing, that’s unlikely. The main problems are that the
flux generally costs more than just boric acid (a minor
consideration, as both are cheap), and more importantly, the alcohol
boric acid solution wets the metal, and coats it uniformly. The water
based flux tends, due to surface tension if not absolutely clean, to
not wet the whole surface, instead balling up/pulling away from
areas. You get around that by brushing more on while it’s hot enough
to keep it there, but that’s more work. So while the flux works fine,
in general, the boric acid/alcohol mix may be a bit more reliable and
foolproof. But YMMV, as always.