String Y already gives free iframes, adding up the instant buffed E would force you to hold block. Because you are invincible from everything, you can just dash m2 and easily get a free combo. The skill lasts almost as long as the CD, allowing others to spam it repeatedly.
#Disable m12s and skills during string Y iframes.
stark flame
valid basalt
I agree. Main benefits of the skill should come from the new skillset, not the iframes
However after implementing your suggestion, if implemented at all, string users will only be able to run and not attack opponents camping on them. With most slayer skills allowing easier and fast movement for slayers, String users will have a harder time escaping campers.