Completed my upgrades on the SL8 this morning. At the moment I'm lacking the motivation to hurt myself much in the mornings, so I've been doing Z1/Z2 with Coco for the majority of my morning rides, and will probably continue to do so until Christmas. What is the best bike to do distance-based upgrades on next? (I'll switch to the Cadex TT bike for when I do workouts.)
In terms of road bikes there's not really anything better than the SL8 to upgrade that uses distance for upgrades. An upgraded TT bike is handy for TTs in ZRL etc but uses time rather than distance for upgrades as you know. The Specialised Aethos as a pure climbing bike may also occasionally come in handy as well but that uses elevation gain for upgrades (as do MTBs).
I suppose you could do a gravel bike?
Other than that, the only potential benefit you could look at is going for the Specialised Project 74 Halo bike which I believe is the fastest bike in Zwift on the flat when fully upgraded (but crap uphill!) - but you'll have to first fully upgrade two other Specialised Bikes and then do another 8,000km on the Project 74 to unlock it - a very long term undertaking!
A lot of it is probably in the head and we are not all the same.