Does the bike start and idle fine? Rev out clean when just sitting there? If so, it is very possible your CDI is fucked. I have seen them on more that one occasion cause this type of issue.
You really need to get the bike warm enough to get color on the plugs. I would also start at 300 main and 27.5 pilot. Get it into 4-5th gear if possible. Get it good and warm. You want the plug strap to be cardboard colored.
The only other thing that I can think of is the CDI. I have seen several CDI that will allow the bike to start and rev clean but fall off under a load. Since you’re just starting to get moving in 2nd and 3rd I can see this being a possibility.