Hello Kendalizor and welcome to Apple Support Communities.
I see you're trying to customize what holidays appear in the default Holidays calendar. You're correct that this is an all-or-nothing proposition. If the Holidays calendar is enabled, it will list all holidays contained in that calendar. These are not editable.
However, there is a way to customize this through a subscription to a 3rd party calendar. First search for a calendar that more suits your needs. For example, if I wanted an option that only showed me federal holidays in the U.S., I would search "subscribe calendar US federal holidays."
Once you've found a calendar that better suits your needs, use the info here to add it to your Calendar app.
Use iCloud calendar subscriptions - Apple Support
Cheers.