How can I do this without running into infinite recursions?

I have packed rbds interacting with a flip sim. How can I use the position of the rbd objects to update the position of the wavetank? I want my object to be always centered in the middle of it. Since its movement is unpredictable I can't output a path or something.

Timeshift (to get the last frames data) doesn't help unfortunately. Wonder whether I can use a solver with "previous frame" somehow, but currently struggle with this idea. Is there a way in an expression to get the last frames result, so that there is no recursion problem?

Hope somebody has an idea.

Cheers
Tom