In case it’s helpful to anyone in the future, this is my strategy that has worked for family member in the past consolidating/cleaning up multiple sub calendars in Apple Calendar on Mac:
Calendar > Preferences> Check option to view all events in year view - this leads you identify which months have data.
Hide all but one sub-calendar - look through the year view to identify which months have data that needs to be saved.
Month view - If I find data in a month that needs to either be moved to a different calendar or deleted: if you are only viewing one sub-calendar, you can CMD-A to select all events in a month, right-click and move to a different calendar or delete.
In my case, I have 17 different calendars. Every one has something in every month and so far every calendar has something in almost every day. A typical day has between 15-25 separate events. All the data needs to be saved. It doesn’t make it into my calendar unless I refer to it on a regular basis. I need to keep the calendars separate, some represent work items that I may have to document for up to 10 years in the past.
Your solutions are probably more helpful for folks with far fewer calendars and events in each calendar.