My suggestion is to add an autocomplete feature in the categories select component.
Currently, the dropdown component that exist in the "suggest new idea" form does not allow users to type in it and make the finding and selecting an easy process.
** Please click on the gif below to see the diffrence.
This is an example of the current implementation of the drop down:
https://codepen.io/AvivHDR/pen/poWzBOd?editors=0010
Try to experience it yourself by finding all the options that end with the letters "er".
An alternative implementation would be this one:
https://codepen.io/AvivHDR/pen/dyVbLrb?editors=001
Here the user can type "er" and all the relevant matches directly appear.
As you can see, in a senario where the user need to choose an item from a list with dozens of options, typing inside the dropdown makes it much easier and intuative.