[GRLUG] Move mail from Time Machine to Thunderbird on Linux

[Grand Rapids Linux Users Group]

My Mac (early 2011; Sierra) hard drive finally died. So I'm stuck with 
what I have on Time Machine as I move to Ubuntu and Thunderbird. 
("Inherited" the Mac from my wife when she got a new one in 2017.)

When I look in Time Machine at Library/Calendars I see 8 "calendars" 
directories with "Events" sub-directories. In four of them, the Events 
sub-dir is empty.

Three of the others just have a few events (11, 127, and 2300) linked to 
a short time period (Jan 2019) or a certain day (16 Sept 2017 or "Thu". 
The  8th calendar has 35K items from 16 Sept 2017 (when I started using 
the "inherited" Macbook Pro) to the present.

That's all background.

Thing is, each item is an .ics. It looks to me like Thunderbird in my 
new Ubuntu box treats each .ics as a separate calendar. So I don't think 
I can just import 37,000+ "calendars" into Thunderbird.

Apparently the "right" thing to do is to "export" from Mac calendar to 
iCal which, I gather, creates a unified .ics one can import to 
Thunderbird. That train has left the station with the death of my Mac 
hard drive.

I suppose I could grab my wife's Macbook Pro on which I have an account 
and "restore" from Time Machine to Library on my account and do the 
export. But she's kind of proprietary about her machine...

Does anyone know of any other way to get from the 37,000 ics's in Time 
Machine to Thunderbird on Ubuntu?

Thanks.