1. Good question, & perhaps the key wherein the answer lies: I have, since late '07, developed the amount of calendars I use currently to 20 different Calendars under the heading "On My Mac", with 12 "Subscribed" (10 chkd), under the "Other" Heading below.
This is because I used to have this many when Calendar (then called Mac "Agenda", if I recall correctly), historically had only one main "Calendar", with many possible "Categories".
Then Apple at one point, chose to improve (?!?!) the software, so that what were previously "Categories" of the same main "Calendar" then became different "Calendars". Do you remember that?
Now, each of my 20 sub- "Calendars" have different colors to help distinguish the different events I create.
Yes, I'm aware this is like no one else uses Calendar. I plan my whole day, from getting up to going to bed, with the main events of my day and keep track of these as the day progresses, and can see how well I stick to my planned "Events".
So, my "Calendar" is very full of Events. I've been retired for more than 18 years now, and this is very useful to me, and is how I stay as active as I can, including a daily exercise session of never less than 1 hour every day at my gym, etc, etc.
2. I'm not syncing with any other accounts: listed are "Charles iCloud", "Google Calendar", and "FaceBook", but I haven't chkd the "Enable" box on any of them due to problems encountered when I tried to sync with my iPhn 5 a few years ago (another issue).
3. My setting in Calendar>Preferences...>General>Default Calendars: is "Selected Calendar"