These are just my thoughts as I’m no authority on this.
Since you are doing interval training and if you are doing it right then when you finish a workout you should be basically spent. If not your workouts are not challenging enough. There is a bias function that will allow you to either reduce or increase the intensity of the workout, in other words allow you to adjust your ftp on the fly. Do this sparingly if at all.
When you finish the ftp builder after 6 weeks and do the ftp test you should be showing a much higher then 150 ftp if you still now have the energy to continue riding after you do all of the workout in your current plan. So using the new ftp should make it a lot harder at least in the beginning of what ever training plan you select next.
I have been on Zwift for a little over a year. Started this as an intermediate beginner if there is such a thing. For the first full year I did a few few races (LOL), a lot of group social rides and received all of the route badges, completed all three of the challenges and got the Tron. About 5000 miles and 385,000 feet of climbing. But I had not done any structured workouts at all. So I was in a decent state of fitness (age 62, 61.7kg). My ftp went from 130 to 190 but has been stalled at 190 for about 4-5 months.
With a reasonable degree of fitness, I then started my first workout program, I’m now in week 6 of the Build Me Up program. With my ftp set at 190 where it has been for a while, I can tell you that I struggle to complete most of the workouts in the program and have cut way back, but not totally eliminated my riding outside of the workouts. Mainly because I want to put as much as I can into the workouts and secondly I don’t have the energy to do my typical 150 miles a week while doing these workouts.
All we can do is start from where we are at and go from there. I applaud you for putting in the effort and wanting to be challenged. So, without knowing anything about you except what you have posted, and not knowing your goals I would say that most likely you are not challenging yourself enough in the workouts you are already doing.
I don’t think it’s a good idea to do ftp tests while in a program because the workouts are designed to increase your ftp by scaling up the intensity as the weeks go by. Finish what you are doing, take an ftp test and give it your all, then select another workout program and concentrate on getting all of the stars available and doing easy recovery rides as extras. Make sure you have the correct ftp in Zwift when you start your next workout program. I assume that you are using a Zwift supported trainer and have it calibrated correctly.