Don't Laugh. Here is an example of an excellent 21 inch (53.34 cm) STEP THROUGH FRAME. Yeah, it is an electroforged Schwinn of course.
THIS ONE IS FROM THE LARGEST SIZE (21") SUBURBAN, and as you know the Suburban featured the Tubular front fork of the Continental.
(trading old stuff on EBAY who parts out schwinns has a great reputation, I've purchased items from him)
Anyway, it is just frame with the fork......No Kickstand, No crank, No Stem, No Seatpost, No Headbadge.....you do get the Chainguard..
You wouldn't care about the kickstand, as you'll save 2 pounds there, and you'd install a lightweight 3 piece bb set-up that adapts to the ONE PIECE's Housing.
No Joke, you won't find a more solid and stable STEP THROUGH frame than this SUBURBAN schwinn's. You could probably find one in your neighborhood for the same price or less, but heck for ~ $50 total shipped to your door, ah ya can't really beat that, and it is already stripped down of the stuff that you wouldn't want anyway.
WALD sells New 13/16" diameter seat posts in several lengths with the modern 7/8 top mount. You can get a Schwinn Seatpost clamp that is nice for less than $5.
The ALLOY (S) stem from a seventies era CONTINENTAL is lighter than the forged Steel traditional stem seen on Seventies era Varsity, Suburban & Collegiate etc.
'72 Suburban (JH serial indicates Sept 1972) Green 21" bare full frame including the tubular front fork(Suburbans & Continentals had this fork which is much lighter than the Ashtabula Forged steel Blade Fork on VARSITY/COLLEGIATE/BREEZE...etc and many many others)
https://www.ebay.com/itm/313662097423
I just don't think that you are going to gain much IF ANYTHING with most other aluminum or steel STEP THROUGH or MIXTE frames. Yes, you might save a couple of pounds in FRAME weight, but I contend that the SCHWINN frame is better by far all of them except perhaps some mixte frames which will be as solid and stable as the old Schwinn electroforged Step through. The great benefit is the strength and size of the (21) frame for a good size adult male. Compareably it is both longer wheelbase and larger than similar modern sized equivalents. The frame geometry is such that it is very stable and the angle of the seat-tube lends itself more to accomodating taller and larger Male adults if needed. THE GREAT NEWS IS THAT SCHWINN made a (19) step-through frame that also is comepareably larger and longer wheelbased than similar modern sized equivalents. Schwinn also made a small (17) step through version also.
I certainly would consider that as a possibility. Just obtain a SCHWINN bare frame, or if you find a free complete bike, etc.....STRIP IT DOWN SO IT IS JUST THE BARE FRAME, NO KICKSTAND, USE LIGHTWEIGHT 3 PIECE MODERN CRANK SET-UP, USE VERY LIGHTWEIGHT ALUMINUM 700C (622mm wheels) with perhaps 700C x 35mm tires......... You certainly want to INSTALL modern sidepull calipers IF YOU DON'T care for the old vintage WEINMANNS.
I would also while the frame is bare, I would prep and paint it a color of my own choosing......or have it powder coated assuming that color was a possibility. Heck, you don't need to let anyone know the frame came from a Schwinn if you don't want to. Anyway it is just a thought. I do think that if you judiciously select wheels and parts that you can easily get your final finished project weight below 30 pounds. Yes, perhaps that still sounds incredibly heavy to you, but practically speaking that is light enough to do everything you'd want to do, unless you race in Sprint triathlons or aim to maintain a 20mph average ride pace with the "A" group from the Local Bike Shop. Short of that, assuming you gear the bicycle decetly, and wide ranging enough, you'd have no issues going coast to coast and back with that step-through frame.
Yeah, step through frames & mixte frames can be a P.I.T.A. to mount on typical exterior rear car racks, without a specialty adaptor bar to aid & allow mounting.