Amy,
I've been watching these answers because I knew your ask was a tough one. Even putting budget aside, no quick easy solutions.

I assembled a 26 lb step through for the better half, starting with an old big box store aluminum hybrid bike. Stripped it of the cheap heavy everything and went with reasonably light replacements. Only disc on the front, it's a mullet. I didn't keep close track but probably put $300 of new parts plus wheelset parts I had on hand, say another $200ish. Spray paint and decals too. Trying to get near your specs is not a trivial project.