Hello Lixin
It is too early to have information on the strike. Trenitalia is publishing their backup timetable only a few days before, if doable at all. It can be that the EC connections are running while the rest of the traffic is cancelled, as Trenord will be on strike that day. And, Trenord tickets are not valid on the Eurocities. I do not know of a bus company, other than Flix bus, that serves Chiasso to Milan directly.
Kind regards,
Roland