I don’t see any reason why you can’t continue to increase your FTP, if you’ve only been training a year or so. It’s just going to be harder.
I think it’s a bit of a myth that skinny riders can’t be fast on the flats. Remember Chris Froome keeping up with Peter Sagan in that TdF crosswind attack?
Maybe instead of thinking about weight, think about strength. I bet riders like Froome have great core strength, despite their stick insect look. My big weakness, at 189cm tall and with a past injury, is my lower back.
Do more focused core work and over the off-season, maybe a few weights - see Coach Chad’s recommendations.
Also think about bike position and getting more aero - something that better core strength will let you hold for longer.