Shuesawhau82 You can also take an IC21 train to Lugano and a RE80 train from Lugano to Milano Centrale. You have a connection every two hours.
No seat reservations are required in IC21/RE80, it is only recommended in the peak times (for example at the beginning and the end of weekends/holidays). You can use your Swiss Travel Pass until Chiasso.
If you take a regional train RE80, you should buy an Arcobaleno Transfrontaliero ticket on sbb.ch or in SBB Mobile app only from Chiasso to Milan. It is not necessary to buy this ticket in advance, please note that it is valid 4 hours.