When is the Right Time to Add Volume?

Some background: I completed SSB MV 1&2 before both of my 2018 and 2019 seasons, each continuing into a MV build plan. My FTP gains each year were around 30W (about 3.45W/kg to 4.2W/kg over those two years). I only just started racing in 2017 and am almost 30 so no real long history in the sport.

I’m about to complete SSB MV1 and had to back my FTP setting down because I was failing OU workouts and even the higher % SS intervals were becoming brutal. I tested at 299W in the off season and 308W at the start of MV1. I set it to 303W for now and have one week before recovery week.

My question is: Should I have been doing the high volume plan or a modification of HV with less TSS but more than MV? Talking with my teammates, they are thinking I should be doing longer SS days (HV) to really build muscular endurance. Honestly I have no idea how my body will react to this next round. The workouts I’ve been doing in MV are hard, sometimes brutal, but I always feel okay on back-to-back days. It’s not like I’m failing from fatigue. I’m failing OUs (Tunemah and Palisade) because I felt like I was in VO2 territory and it’s why I dropped my FTP setting 5W.

When would someone know it’s time to increase volume? Like I don’t want to get through MV 1 and 2 only to find out it didn’t get me anything and I should have done HV.