Cool to see you’re anxious to get going, I was the same way. I like having some structure or a goal to shoot for when riding, specifically when indoors at this time of year.
Now I maybe wrong but I think you’re right with your comment you’re over thinking it. Once you do your FTP test and set your zones the weekend Zone 2 ride will be exactly that - in Zone 2. My first Z2 ride I was under the forecast TSS for the time, the following weekend I was bang on. What was the difference - well you’ll note you’re given a zone, the first weekend I rode towards the bottom of the zone while the following weekend I rode aiming for the top end of the zone and was bang one which I notice has an impact on TSS.
I had a similar experience yesterday I posted about, had a 3hr SS group ride and it was my first ride outdoors & didn’t realize how difficult it would be to modulate power compared to indoors that I ended up achieving the prescribed TSS in just over 2hrs. - power numbers were in the right zone though. Not wanting to accumulate too much additional TSS unless in Z2 I called it quits as I still have a 2.5hr Z2 ride today. It’s my understanding any additional accumulate TSS should be only in Z2 so it doesn’t affect power and recovery for future sessions.
That’s my understanding.