7/64 amsteel is plenty strong (even if knots are used)
a single strand of amsteel and a knot will be lighter than whoopies, (you use half as much line this way, so the line portion would be half as heavy) straps would be the same of course.
tying amsteel directly to your straps is workable, but will wear through the strap eventually. for this setup use a slipped double sheetbend, lighter users may get away with a slipped single sheetbend.
if you get tree straps with hardware in the end (like a small welded d ring) that will add about 1/2 oz total to your straps, but you don't have to worry about the line wearing through the strap over time. if tying to a ring, use a slipped buntline hitch.
a clove hitch may work, but you'll want to make it slipped as it will tighten down and be very difficult to untie otherwise. slipped sheetbend is very similar to a slipped clove, and more useful imo.
you could also get away with something like dynaglide whoopies that may be even lighter than this (not sure), but that's some thin cord to hang from, and it only comes in dayglow colors (not saying it ain't strong enough, but check it often for signs of abrasion)
if you want to make straps with a metal ring i could send you a pair of welded triangle ring (8g each), or just sell you some straps that are already made this way.
i wouldn't worry about whoopies wearing out in the field, and if they did you'd have plenty of line there to revert to line/knots.