The greatest gift Palm's WebOS gave us was the unified calendar (and the greatest curse was those ads with the lady on the rock). I agree that having too many calendars is problematic, but I think a handful of calendars, shown together, and shared as necessary, is the answer. I also am troubled by putting personal stuff on a work calendar, because I think that's a line that should be rarely, if ever, crossed. If you were unsatisfied with your employer and interviewing, would you put interviews on your work calendar?
Another commenter mentioned it, and I agree, there is something to running a calendar server of one's own, if one is capable of doing it. I've been trying to scratch this itch for years to little success, but feel like it gets closer all the time. Things like NextCloud make it even easier still, but lack the way I want to deal with groupware (service accounts). I'm hopeful that Apple Family Sharing can solve some of this for me and my partner as we both use iCloud calendars today, but haven't set it up yet.
Over all, I agree, and wish I knew the answer, but I don't.
Android does have a unified calendar that every app push into.
It has two problems (that are both dealbreakers to me): it's not easy to make it push the complete schedule into any other calendar (if it's possible at all, I never managed to), and the apps are often not reliable (probably because they don't want to allow a centralized calendar, the MS ones are serious offenders here).
I agree with your overall sentiments in this matter. I am planning to setup a NextCloud server since I have a synology NAS that is under utilized. Partnering that with an RPi running CF Argo Tunnels would securely get it remotely accessible. SO..... I am just lazy.
EDIT: lazy mostly because of the hassle of importing data from my work Google Calendar, and my Apple personal/family calendars.
Another commenter mentioned it, and I agree, there is something to running a calendar server of one's own, if one is capable of doing it. I've been trying to scratch this itch for years to little success, but feel like it gets closer all the time. Things like NextCloud make it even easier still, but lack the way I want to deal with groupware (service accounts). I'm hopeful that Apple Family Sharing can solve some of this for me and my partner as we both use iCloud calendars today, but haven't set it up yet.
Over all, I agree, and wish I knew the answer, but I don't.