I'm a beginner, so I don't really think I can do it by myself. Based on your response, I take it that it's not provided as a built-in option and it requires lots of effort or there's a commercial module. Hmm, in that case, I think I'm just okay with the current possibility with the selectbox and keep learning about processwire as much as I can as there is no urgent timeframe. Thank you for your time, Bernhard! ?
Edit: Not really thousands of, but dozens of options max.
I'm striving to create a small e-commerce website for myself. And thought, checking subcategories would be easier on the eyes than selecting them.