First of all, sorry for the delay in answering!
Secondly, I think you are the first person to print Thor in resin! You must have a huge resin printer! Hahahaha
As you say, usually a resin printed piece is heavier than a plastic one (especially if the resin one is completely solid). This can be a problem, since the motors are sized for the weights of the plastic printed parts and it can happen what you say, that the motors do not give enough power to move the arm.
There is a user who had a similar problem to the one you mention about the second articulation and he designed some supports for the lower part that I think solved his problem. I link the post here in case it helps you.
Regarding Asgard’s doubts. In principle the Art2 slider should move the two motors with printed gears that are located at the bottom. The Art3 slider should move the motor of the pulley that has the open belt.
Both movements are independent, so it must have been a coincidence what you say.
As the “stable” Asgard version has no direct kinematics implemented all the movements you make of the joints are independent. The only exception are Art5 and Art6, which as you say are related, since each of the sliders moves a motor that is connected to the semidifferential.
I hope this clears up your doubts and let us know if you have managed to solve the problem with the second articulation!