Samsung Galaxy S — The Challenge? One "Agnostic" Calendar

I'm VERY new to Android, from what I gather I also lucked out picking the Samsung Galaxy S8.
I have just rebooted an LLC that offers"personal and/or business" Network, Hardware, Software and "mentoring" services.
I've found a great market for it and after ~40 years or so in IT, I like doing it as I ease into retirement.

• OFF-TOPIC BACKGROUND *
  I truly mean a Legit 40 years, "plucked" or "drafted" in 1976 at 20 inly 2 years of college, 4 months later I found myself working in a five-sided building in the US Capital. (Yes, the US DOD+NASA, Trust me, no one was more shocked than myself. This was the wild wild west of ARPA morphing into DARPA net)
  Point being I'm pretty Tech savvy, next 28-29 years were CAO & CTO positions at GE and IBM.

• -------------------------- *

  This business means supporting "lots" of disparate platforms and OSes.
  I have, up until just recently always been an iPhone person. This business has forced me, initially at least, into the Android camp.
  The once "Hemroid" now Android, "Me Likely" but it has also created a bit of a problem when it comes to scheduling appointments and events. (Calendaring)
  Things were somewhat manageable up until Apple's two factor authentication.
  I need a calendar that can sync ALL my devices, Google and Apple are "no go's" they just don't play nice, it also can be seen as "endorsing", which I will not do.
  I thought since all of my cellular devices use Verizon, maybe that could give me a nuetral solution, NOPE. I next thought Samsung, again NOPE.
  I could possibly use my hosting solution and kludge together a CALDAV type, but that's going to be a "kludge" especially when syncing between Android, IOS, Windows Mobile, MacOS, Windows 7-10, Tablets and possibly Linux, (although I dont see a lot of that.)
  It may be that Outlook, might be as close to an "agnostic" solution as I can get. Not so sure yet, it looks as though functionality varies espcially on the mobile side. I'm not at all sure if each device's "calendar" is only local and how "syncing" between mobile and "desktop" devices would handle that syncing.
  I really don't want to make syncing have to involve multiple and/or different cloud or even local apps.
  I'd love to hear from ANYONE that can recommend a mobile and "desktop" calendaring solution for syncing, Google, IOS, Android, MacOS, and my hosted domain's accounts "Appointments, Events and Reminders (that's only a Nice to Have)
  Google and Apple just don't play "nice",
  Since Verizon is my cell carrier that would be great, but haven't seen that as a workable solution, yet.
  I need to have a neutral/agnostic calendar that can work with Android, IOS, Mac OS, Windows on both mobile and desktop platforms.

Thanks to ANYONE in advance for any suggestions, thought, ideas.
Very Sincerely,
Peter