Lightweight Bicycle suggestion

I've never seen a lightweight beach cruiser.

To get lighter mountain bikes will cost proportionally more money. Every ounce lighter costs $$$.

OK but my mountain bike is over 30 years old, steel and at 35 pounds I can still lift it on to a rack. Just riding off road should develop your arms ability to lift it. Never seen a light weight beach cruiser and generally they are good only for circling the campground i.e. junk. If you are truly riding off road a mountain bike is essential. Otherwise consider a flat bar road bike. Mine is carbon and 23 pounds.

How much is too much weight? I suggest you weigh your current bikes and compare to various others you think weigh less. Until you're in the thousand++ price point, there is little weight difference.

You may find a difference of 5lb or 10lb tops. And buying a 30lb bike vs 20lbs is not cheap and highly unlikely. But even at 10lb reduction, does it really matter on a bike rack? Even on the back of a camper? IMO no...

25lbs would be a common lightweight aluminum cruiser FYI. Getting below that..you'll pay big.

Lightweight means more expensive.

If your mtn bikes are steel framed, an aluminum frame bike would be the next step up in lightness.
The next step up is carbon fiber frame, more expensive.

Without knowing which frame material your current bikes have, it's hard for me to make a suggestion.
Also, do you still want a bike to ride off road?