ZwiftInsider is a great resource for picking the fastest bike on different courses.
S Works Venge with the DT Swiss disc is faster on flat courses, the Aethos with Mielensteins is quicker uphill (n.b. absolute fastest bike may have changed since I last checked but they’re both faster than the Tron for those uses). But the Tron is a better allround bet for mixed courses.
For the ZRL I usually pick based on what’s going to be most critical for my race plan / my abilities on a given course (I don’t usually struggle to sit in on the flat so I usually only take the venge plus disc for the dead flat sprinter courses).