If I understand right, instead of showing all categories in the map filter checked by default, you want to start with 1 category checked by default. Is that correct?
If I’m right, please not that this is a complex JS change. I’m not sure we can guide you to do the change since it’s a modification that has to be tested to confirm it works. But I want to make sure I understand what you want first.
Since it’s most probably a custom development order, you can send me the reply at [email protected] and I’ll follow up with a custom work offer as soon as possible, if interested in custom work of course.