IDD. Just responding on the point about Strava being the reason we can’t have particular features.
I do think that if you “join” a pace partner group, the default route you’re on should (optionally?) adapt.
But it’s still suboptimal. You can’t immediately start following a PP when you get close. The potential problem there is clear, if, say a PP group catches you just before your next scheduled turn and you get pulled off route.
So as you say, it would have to only take effect after an amount of time. Until you get to the first multiplier perhaps. In the meantime you’re still having to make manual turns, with the risk you might get it wrong. The UI could be improved to help with both issues I expect. The options could be displayed as “Follow Pace Partner” and “Follow Route” (if one has been selected and you’re not free-riding anyway).
But yes, the simplest thing that could possibly work is starting a new activity. Not as much of a PITA now that we can end a ride without exiting Zwift. 