>Do you think that the Gorda/ Juan de Fuca plates being locked solid

This is not an accurate depiction of this margin though. Cascadia appears to experience both large slip events, but also so-called episodic tremor and slip (ETS) events (e.g., Dragert et al., 2001, Brudzinki & Allen, 2007), so it's not as though subduction is completely paused along this margin between megathrust events.

> might be creating a delayed onset of volcanic activity? Like we know the last mega thrust quake was 200-ish years ago, could it be that the crust pushed down in that event just hasn’t had time to really melt much yet?

More importantly though, and similar to the original consideration of OP, this is a ridiculously short time frame to consider when we're thinking about processes that when discussing the relevant rates we typically average over 10^(4) to 10^(6) years.