Hi Kim,
Welcome!
You're right, a <16kg boat will be much nicer to carry and surf waves with.
If you're 100% ok on flat water and 95% ok in medium swell (that means many different things to people), you can definitely go 1 step down in beam safely and possibly 2 depending on your ambition, determination, and how long you're willing to stay in smaller conditions.
Going from 50 to 48cm should be no problem. In flat water you will adapt (and possibly out grow) very quickly, but you will notice a lot of enjoyment increase just from a composite boat (stiffer) that is reasonable weight. 48cm should allow you to go out in the ocean relatively soon as well. You may swim a few times in bigger stuff, but should be able to adapt quickly.
If you go down 2 steps to a 45cm boat like a Swordfish S, that is a much larger step than a 48cm boat. Although I consider the SF-S very stable its all very personal. I like to challenge myself and was paddling 3+ days a week, so mastered it relatively quickly, but im younger, and if you're not paddling at least 2x a week (and ideally 3-4x a week until you're comfortable in waves) the 48cm boat will be more enjoyable.
Overall I think you will be happy with a 48cm boat like a Fenn XT (ideally the -S model), Stellar SR (gen 2 only. Gen 1 sucks), or others. If you like riding waves, I'd avoid boats with no rocker (V10 Sport) as they do not pick up and surf waves as well as boats with more rocker (XT, SRg2, New Nelo's). Conversely, if you intend to do mostly flat water paddling, a low rocker boat like the sport may be slightly faster. Also, 48 cm boats are in generally high demand, so if you buy used and outgrow a 48, you can likely sell it for the same price and upgrade next year at no loss.
Edit - one last thought, since you're a woman, if you're smaller/lighter, a boat with too much volume will feel 'corky' and unsettled because you are not displacing enough water to reach the design waterline, which may result in the feeling of great instability. If you try out a 48cm boat but it feels very tippy, be sure to seek out a low volume boat designed for smaller people like the Nelo line in "M" versions or others like the V10L (although this is elite skinny at 43cm). A Nelo 520M or 550M would definitely be worth seeking out for a test paddle.