Not sure I know enough to know, but I think your analysis is spot on. I believe that your 4-6 hour effort at 65-75% of FTP helps build endurance, but once you have that, your training should have more focus on raising your FTP.

In my little pea brain, periodization says, "If you do long slow rides, you'll be a long slow rider."

It's OK to ride too hard, and/or too far, in training. That's what cell phones are for.

I always prefer quality over quantity, although a mix of both for IM is a good idea.

What I did for my last IM:
Shoot for a .80 IF for each hour of the L ride.
This is pretty 'easy' to do for the first 3 hours, and becomes progressively much harder for the last 2 (or more).
I didn't do any intervals, specifically, but rode rolling hilly courses, and punched it on all the climbs, and then kept it solid and steady on the flats.
The first few times I did this, was able to hit the .80 for each of the 3 hours, then a little less for the 4th, and then cracked & rolled home as best I could for the last 1-1.5 hrs.

Eventually, I was able to hit the .80 for the whole shebang, typically 5-5.5hrs - which makes riding at .70-.75 on raceday feel like a recovery ride.
YMMV.

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float , hammer , and jog

You're on the right path. You don't have to ride 5+ hours very often once you're all set to hold hold aero that long and if you're a pretty fast IM athlete. Often I find that the weekend "long aerobic ride" is the most overrated session of the week. Something like Tempo's @ 85-90% FTP for a couple of hours or like you said, 2x30@ 95-100% over the course of 3 hours is plenty.

It's#2
Edit: you're lacking the muscular endurance to hold aero for 112. Pacing may have also been off.

You want to do long rides with long efforts at or above 230 Watts IN AERO.

BTW, plenty of people see a decrease in watts in aero, so riding at the same power as last year is an improvement. I bet you went a lot faster.

You should really only 1-2 long rides at IM power. All your other long rides should be way harder. Most IM athletes think they need to do a ton of long rides to get good at IMs. It looks sexy to ride 250+ miles on strava but in reality if most of that is from a ride that is 6 hrs at 65-70% is a waste of time. Ride 3-4 hrs at 80%+. Even better if you do something like 10x10min at 90%. Keep overall volume the same but up intensity on your rides compared to last year. Those long 6 hrs take a lot of you and chews up your day.

Didn't we already have this arguement? ;)

Thanks everybody for chiming! Really helpful to get some extra opinions on this.

I like the way you think Karl. Hadn't really considered that a many riders aren't able to put out the same watts compared to their regular road position. My new bike/position did certainly pay it's dividend (4:43 vs 5:21 on the same course, slightly less windy this year)

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Pain is temporary.

I have to admit I blindly followed the TrainerRoad plans for the past two years, without applying the training principles I learned from books and online. Looking at those plans now, I'm kind of surprised how similar all the Sunday rides in the TrainerRoad build and specialty plans are. All long and slow rides with an IF of 0.66-0.67.

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Pain is temporary.