I highly recommend MS Outlook. It is very powerful and very scriptable. Keep in mind that Outlook provides 5 major "sub-apps":
- Calendar (much, much better than Apple Calendar)
- Contacts (again, much better)
- Notes (I prefer Evernote)
- Reminders (available on all of the above)
and all of these may be assigned one or more "Categories", which is just another name for tags. Very powerful.
I've used Outlook 2011 for years, and have just upgraded to Outlook 365 now that I'm running Mojave. So far I have found Outlook 365 to be just as scriptable as Outlook 201, with one exception: You can't use a script in Outlook 365 Rules. However, there is much you can do with OL Rules without a script. See Create a rule in Outlook for Mac .
So while you can't us a script in a OL 365 Rule, you can easily setup a KM Macro with a periodic trigger that runs very often (I have one that runs every 5 min with no impact) to do whatever standard processing you may have.
A great advantage of OL is that you can setup your email accounts using Exchange Server, either hosted by your company, or by a hosting provider. This enables all of your OL data (email, contacts, calendar, tasks, etc) to be quickly sync'd to all devices that use the same ES account.
Of course, OL can also use Pop3 and Imap accounts, as well as multiple Exchange Server accounts.
If you have any specific questions, please let me know.
If you have a choice, I would recommend going with either Sierra, or Mojave. High Sierra provides a lot of pain for little gain. I just upgraded from Sierra to Mojave, and it went very smooth.
Outlook 365 should be able to easily handle that.