Tmk63
You need only a ticket from Como to Chiasso, your STP is valid after this station. It doesn’t mean that you must change, you can stay in the same train RE80 and continue to Locarno. This ticket costs less as 2 CHF in the SBB app.
For the scenic route, the Centovalli, the STP is valid, but seat reservations are recommended, they are only available on this site:
https://www.vigezzinacentovalli.com/en/