It sounds like you are resolute that the trellis is the way to move forwards. Have you already searched the forum for the term “apple trellis” and checked out how others have constructed theirs, the things that worked out well for them?
I agree with @Masbustelo (a coffee reference?) that fence strainers are the way to go for tensioning a trellis wire. I have used them before for that purpose and have already laid in a quantity for tensioning the wire in my next trellis project.