I'll also add that if your goal is improving FTP on 6-8 hours a week, then polarized is definitely not the plan for that, as so much specific FPT training involves training in that dreaded zone 2 of the polarized 3 zone plan.
You can conceivably do 3-5 days a week of 20-60 minutes of tempo/sweetspot/threshold (depending on your experience and recovery) which is precisely the type of training that will most improve your FTP in a shorter time frame with shorter training durations.