How long are your long outdoor rides? And what has made you drop to the 150 mile course in previous years? Is that a decision you have to make beforehand or you just drop down on the day if you’re halfway through and don’t feel like you can handle it?
I think you’re on the right training plan but have a pacing issue on your outdoor rides. Those VIs are very high for solo efforts. For context I typically have a VI of 1.2-1.25 on fast group rides and road races with a big pack where there’s a lot of easy cruising in the peloton interspersed with some very hard efforts on hills, when on the front, when somebody is attacking, etc. Those are hard days, I can cope with that kind of variability up to about 4 hours/100 miles, but no way could I do it for 200 miles and 10+ hours. For a long solo ride my VI is typically below 1.1, if there’s nothing too steep then it would be below 1.05. But those VIs are a function of pacing not fitness. I can ride at 1.4 or I can ride at 1.01, it’s a choice. To get to 1.4 you must be hammering it and then taking it easy. Don’t!
And yes, for a race that long you don’t need to go above threshold so it is all about raising FTP and increasing endurance. But working above FTP is also a great way of raising it, so don’t make the mistake of thinking you should only train below threshold.