I'm struggling getting the custom resize / move window to do what I want (even after I understood that that I need to check the "Really" boxes).
It would be much easier if there was a way to trigger a snap area instead, either by adding the option to the "Custom Move / Resize Window", or by a separate "Custom trigger snap area" action where I can pick my window (by title / app / whatever).
That just gave me an idea on how to get the custom move/resize to work. I basically replicated the snap area. Instead of fiddeling around with relative screen sizes, I took the numbers from the snap area configuration, moved the window to the bottom left corner, offset it by those numbers and finally resized it with the same numbers. And it worked! 
An action to move a specific window to a snap area would still be a good idea though.
there is n action to trigger a specific snap area, I‘ll lookup the name later (currently not on my mac)
That one only works for the active window (unless I missed something).
that‘s true, but maybe when combined with the activate specific window actions?
Yes, I could activate the window, snap it, and then reactivate the previous window. But that's exactly what I'm trying to avoid.
Being able to snap a window without activating it would be a nice feature (and asfaict, easy to implement).
Anyway, I got it working by replicating the snap areas to a custom move/resize window (using wildcards in the window titles - thanks for implementing that hidden feature!), so no hurry for me.
I can see three ways to do it:
Am I missing something? If I already have a snap area and want to resize it, I have to manually guess the new x,y coordinates and also guess/play around with the x/y dimensions.
It would be easier to either have handles to resize existing snaps or use an open window as the new template to update existing snaps.
[Update] Found a simple-ish workaround