1.16.4 Port (Not an annoying request, I want to port it)

[Oakleafknight06]

I am trying to start work on a port to 1.16.4 and I have a few questions:

1. I am having a few issues getting this to work in my IDE (IntelliJ) and it's having some issues, any tips on how to get started?
   Crash report [https://gist.github.com/Oakleafknight06/89bbd471ce73ce877f123d12b1aea3c3]
2. Roughly how complete is the 1.15 port? Looking at the commits and stuff it seems that it's mostly rendering issues due to the rendering changes made in 1.15

[Oakleafknight06]

[11:54:54] [main/FATAL] (mixin) Mixin apply failed mixins.cotton-datapack.json:MixinClientBuiltinResourcePackProvider -> net.minecraft.client.resource.ClientBuiltinResourcePackProvider: org.spongepowered.asm.mixin.transformer.throwables.InvalidMixinException Unexpecteded ClassMetadataNotFoundException whilst transforming the mixin class: [MAIN Applicator Phase -> mixins.cotton-datapack.json:MixinClientBuiltinResourcePackProvider -> Apply Methods -> (Ljava/util/function/Consumer;Lnet/minecraft/class_3288$class_5351;Lorg/spongepowered/asm/mixin/injection/callback/CallbackInfo;)V:handler$zem000$addVirtualPacks -> Transform LVT -> var=factory -> desc=Lnet/minecraft/class_3288$class_5351;]

Another bit of info on what went wrong