I am open to ideas/suggestions on how I could 'prove' I cannot withdraw things myself?
I think it's to late for that now, and people just have to trust that you won't withdraw coins yourself.
One potential way would be to create some multisig setup so that nobody would have full control over keys/coins, or splitting seed words with shamir backup scheme.
Maybe there was a way to create simple script for unlocking coins with some terms but I don't know how to do this.
I could for example claim that I destroyed my copies, but is there a foolproof way of proving somehow a withdrawal doesn't come from me later?
No, as far as I know that is impossible to know.
You could always time limit this challenge and create new from scratch doing it in some better way like I suggested before, but think who would you add and trust as other parties in multisig.