well, it's built. so it's longevity is what is it. I've uses untreated pine/fir in my garden for compost heaps, wire frames, trellis, etc. by the time they're going south, you'll have a more better new&improved idea anyway - and a good excuse to try them out!
as to filling it - few crops require 10-12" of depth. carrots - pretty obvious, eh? the rest, not so much.
had a 14x21' hobby greenhouse - fall/winter grew leaf crops / radishes on 3" deep benches, peas in window boxes.
elevated gardening has one drawback - in sunny especially warm areas, the soil can overheat. now, over-warm soil isn't a problem for beans. it is a problem for peas/tomatoes/peppers/nightshade family. you may eventually need to shade the sides from direct sun.
a "soil-less mix" is good - basically ground up bark+peat/compost. see
for fertilizers, , , , organic is good - but watch out for internet recommendations like chicken manure. fresh chicken manure is extremely high in ammonia - it has to be composted at least a year before use.
for info/help with organic methods:
spun off from the Organic Gardening site after it went 'all commercial'
gardening is a pleasant space surrounded by very deep rabbit holes.....