I definitely don’t want to try to keep three things in sync manually. That said, the current workflow requires entering the dates on Taiga, running a python script to retrieve the dates from Taiga, then copy-and-pasting the python script’s console output into IRC (and repeating the whole process when I inevitably get the dates off by one in Taiga’s poor calendar interface). I think there might be room for improvement. 
My idea of what would be ideal would be to have a bot on the matrix channel push the agreed-to events to a magazine events calendar hosted here on discourse. Maybe with a command like #sched 555 monday. I’m not sure if bots would be able to do that sort of thing though. I’ve never programmed a bot.
I don’t think Taiga’s calendar is really “used” other than as a really odd way to feed information to IRC via an intermediate python script. So, personally, I wouldn’t miss it if we switched to something else.
Just my two cents,
gb