12 weeks does seem to be a very long time to go between tests. Not sure if there’s a rationale behind that but I can’t see any harm in doing one in the lower intensity weeks to keep you training at the right intensities.
I’ve done that plans 2-3 times. Never felt the need for a test in the middle but it’s a good question. The TSS ramps fairly well week to week so perhaps a higher FTP along with higher TSS from week 6-12 might put you in the overtraining zone. Of course, I used to dread the ftp tests but with the ramp test, it wouldn’t be much of an issue
I think that you have up to this point done already so much. You are on top of your game and the workouts are so situated that you can complete them en refine your speciality. A FTP improvement is not the goal, sustain a high percentage of FTP in the triathlon is the goal, when you raise your FTP you sort of start over. Just do the workouts, if your FTP raises then the workouts should be easier but it shouldn’t be hard for a full distance sustainability. If you want to reassess please do.