Hi Bouke,
Redirection
You can ectually change the search button and the ‘return’ key events, on the General Options -> Logic & Behavior panel: https://i.imgur.com/BoiBx2i.png
You can enter the custom relative url on your site, where you want it to be redirected to.
Filtering by custom fields
I think that the problem is, that you are trying to filter User results, which is not possible. The category and custom field filters will only work with custom post type objects (like post, page, products, portfolio etc..). Users and user meta is stored very differently, and unfortunately cannot be filtered.
Ernest Marcinko
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