Your forearm isn't that short to be fair. I'd say there's probably a lot of 6' out guys there that have the same forearm length as you but no one really notices unless you meet someone from this forum, and even then, they probably wouldn't notice
.
Plus, if you do lengthen, being 5'11.5 with a wingspan of 5'9.5 isn't unnatural. It's literally only 2 inches, it's not noticeable and a lot of people have shorter wingspans than height but the average person doesn't check their wingspan.
For e.g. Artem Lobov, a person that should care about his wingspan because of his career (professional MMA fighter) has a wingspan of 5'5 and a height of 5'9 and yes, he didn't do LL...)
https://i.imgur.com/4vhPy3n.jpgBare in mind, Conor Mcgregor is the same height as Artem, but with a long ass wingspan of 6'2.5.
Literally, Conor's wingspan is 9 inches longer and the difference isn't even that obvious, considering it's 9 inches. Obviously, you have to take into account that the arm is not 9 inches longer but you get what I mean, a wingspan of 5'5 on a 5'9 guy doesn't look too disproportional compared to a guy with a long ape index mirrored to his height.
Plus if Artem had humerus lengthening, he wouldn't look too bad imo.
Also; if you think about it, let's say you're standing next to a guy with a 1:1 ape index (same wingspan as height - common, even though higher wingspans are common too) at the height of 5'11.5 , your arm lengths would look similar considering Artem Lobov is not even looking that bad having 9 inches off Mcgregor's wingspan, when Artem has a very negative ape:index.