How to sync Apple Calendar with Thunderbird locally or change alarm display?

I am (well we) giving up Outlook as it was never a good Mac application and is getting worse over time.  Presently I have +10 tickets open some as old as five months+ about problems and no solutions, just loads of useless blah and always a windooz agent, there seem to be no Mac agents (as some of them state).

So switch to  a different solution, Calendar and/or Thunderbird.  The only thing that bothers me is that Calendar has the little alarms that appear in the message center and cannot be put to sleep or delayed like in Outlook where they are big and stay till dismissed or delayed.

I looked at Thunderbird and find the alarms good (design is not top but who cares) but we (there are several of us here with this Outlook frustration) all find the Calendar interface better to work with when setting up calendar events.

We have not been able to find any other solution that would be like Outlook but is not Outlook.  So the idea is to use Calendar to manage appointments and co and use the Thunderbird alarm function.  Only bit missing is syncing them and this needs to be done locally (iCloud and co is not an option because of data protection).  

Does anybody have experience with this and what would you suggest?   Or something that simulates the way Outlook presents alarms with the option to snooze for minutes, hours or even a day or two.  I could imagine there is a small app that reads and works differently with the calendar app allowing a more Outlook way of interating. I like it to be lightweight and preferably free or open source.

Thanks. 

[Re-Titled by Moderator]

Original Title: Sync Apple Calendar with Thunderbird locally or a way to change Calendar’s way to show alarms.