You sound like a good candidate for salvation and being uplifted to the world of side-bys.
Your decision between O/U and S/S occurs on darn near every hunting forum every year, multiple times. I do not respond to every one of them because my fingers would bleed, but I respond to many, including one just last week.
One of the things to consider is that you probably are looking for a 20 or 28 because you want a light gun. Truth be told, most are as heavier or heavier than many side-by 12s (if they are English or Euro). So, why not buy a 12?
I have only seen one truly light o/u
https://www.gunbroker.com/item/960480785 (
@wllm coulda/shoulda bought this one). And it still looks like a fencepost (sorry, but it's true

) and its owner will not look any better for shooting it, whereas someone as hopeless as me can be mistaken for a handsome man-o-the-world so long as I have a true SxS double in hand.
Currently, English doubles in England are going for dimes on the dollar, or simply sold for scrap. Luckily, some folks over here are importing them in batched of a half dozen or more at a time and selling them at very reasonable prices. All you have to do is be patient and know where to look. And, for emphasis, be patient!
Do you reload? What do you want to shoot? Do you like antique tools, furniture, and art?
What sort of statistics do you need in a servicable gun?
Are you handy with tools and interested in investing some sweat equity into a gun?
I realize I am plunging headlong down the road here, and I am not even sure which direction you want to go, but what the hell. Let us cut to the chase.