From what I’ve read and experienced SS TTE and Threshold TTE track pretty well together. The benefits of doing Threshold vs SS is the lower calorie demands and it’s easier to fit into a weekday workout once you start to push your TTE out further. I also think by doing it at FTP you can get a better feel of actually where your Threshold is and if it moves or not; basically using your FTP workouts as frequent little FTP tests. But it might have slightly less specificity to your goals (which IMO isn’t a huge deal during a base phase).
If you plan to do long event I would also consider throwing your SS or Threshold intervals into your Saturday long ride. They are tough at first but once your fitness and fueling improve it is a pretty amazing stimulus to do a 4-5 hour ride with 4-6x20m SS sprinkled throughout.